Experienced attacker David Tubridy hit the crucial goal in Mullingar as Clare powered through to Round 4 of the All-Ireland football qualifiers.
Clare had curiously never lost to Westmeath in a League or Championship game beforehand and maintained that proud record with a gutsy display at TEG Cusack Park.
The sides were level seven times in front of around 5,000 at the sun-kissed midlands venue before Tubridy, who hit 1-02 in total, struck for the vital 47th goal.
It helped Clare to move four points ahead at that stage and while Westmeath reduced their arrears to just a point late on, they were unable to find the equaliser as Colm Collins' side held on.
It's the end of the road then for Westmeath in this year's Championship though Clare march on and will meet a beaten provincial finalist in the final round of qualifiers next weekend.
It was tough on Westmeath, who lost key duo John Heslin and Ger Egan to a black card and an injury respectively early on, while Ronan O'Toole was also black carded in the opening half.
The O'Byrne Cup and Division 3 champions came into the contest chasing their third qualifier win over Munster opposition this summer after wins over Waterford and Limerick.
And they were initially boosted by the inclusion of Heslin, who sat out the Waterford game and only came on against Limerick following hamstring trouble.
Heslin lined out at midfield before moving into the centre-forward position but left the field after a mere seven minutes following a black card.
It was a giant blow to Westmeath and they lost another former captain when Egan crumpled in a heap with a leg injury after 15 minutes.
The team's inspirational leader this year had to be replaced and to make matters worse a third forward, O'Toole, was black carded in the 21st minute.
Clare supporters who made the trip to the midlands could scarcely believe their good fortune and roared their approval as the team raced 0-04 to 0-01 clear.
Westmeath, in the circumstances, might have folded but fought back to level it at 0-04 apiece before edging a point ahead in the 20th minute.
It was an impressive show of character from a depleted side that went through four different free-takers in the first-half alone; Egan, Tommy McDaniel, Callum McCormack and Kieran Martin.
Captain Martin rose to the occasion with three first-half points - and 0-06 overall - though Clare oozed quality at the other end when they attacked.
The 2016 All-Ireland quarter-finalists were level with Westmeath on six different occasions in the first-half with Jamie Malone, Cathal O'Connor and Gary Brennan pinching some terrific scores.
The sides were tied at 0-08 apiece at the interval in a tense and enjoyable tussle that looked set to go right to the wire.
They were tied again at 0-09 to 0-09 before Clare burst clear with an unanswered 1-01, turning the game decisively.
Eoin Cleary converted a 46th minute free and then got a hand to the resulting Westmeath kick-out, directing the ball to Gavin Cooney, who fed Tubridy for a priceless goal.
Westmeath outscored Clare by 0-04 to 0-01 from the 68th minute onwards but just couldn't fight back to parity in an exciting finale.
Clare: Stephen Ryan; Dean Ryan, Cillian Brennan, Gordon Kelly; Sean Collins, Aaron Fitzgerald, Conal O hAinifein; Sean O'Donoghue, Cathal O'Connor (0-02); Eoin Cleary (0-03, 0-03f), Gary Brennan (0-02), Jamie Malone (0-03); Gavin Cooney (0-01), David Tubridy (1-02, 0-02f), Cian O'Dea.
Subs: Kieran Malone for Cooney (48), Eoghan Collins for O hAinineain (51), Gearoid O'Brien for O'Connor (62, black card), Einne O'Connor for C Brennan (70, black card), Alan Sweeney for S Collins (71).
Westmeath: Eoin Carberry; Killian Daly, Jack Smith Boidu Sayeh; Ronan Wallace, Kevin Maguire, James Dolan; John Heslin, Denis Corroon; David Lynch, Ger Egan (0-01), Kieran Martin (0-06, 0-03f); Ronan O'Toole (0-01), Joe Halligan, Callum McCormack (0-02).
Subs: Tommy McDaniel (0-03, 0-02f) for Heslin (7, black card), Sam Duncan for Egan (16), Noel Mulligan (0-01) for O'Toole (23, black card), Sean Pettit for Lynch (58), Tom Molloy (0-01) for McDaniel (67), Conor Slevin for Halligan (68).
Referee: Martin McNally (Monaghan).