Not for the first time in Clare jerseys, David Tubridy and Gary Brennan rose to the occasion with huge displays to propel their team to victory in Mullingar and to within touching distance of the Super 8s.
Level seven times in a tense arm wrestle of an encounter in front of around 5,000, Clare pulled decisively clear in the 47th minute when Tubridy netted and the full-forward from Doonbeg added another two points late on for good measure.
They were all crucial scores but former Ireland star Brennan delivered for his county too, just in a different way, leaping up high twice at the death to seize possession and ensure that Westmeath's gallant fightback came to nought.
Ultimately, time ran out on Westmeath's bid to grab an equaliser and their Championship campaign is over while Clare march on to a Round 4 qualifier next weekend against a beaten provincial finalist.
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.
Clare supporters who made the trip to the midlands could scarcely believe their good fortune and roared their approval as the team raced 0-4 to 0-1 clear.
Westmeath, in the circumstances, might have folded but fought back to level it at 0-4 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 0-6 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 Brennan pinching some terrific scores.
The sides were tied at 0-8 apiece at the interval in a tense and enjoyable tussle that looked set to go right to the wire.
They were tied again at 0-9 to 0-9 before Clare burst clear with an unanswered 1-1, 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 0-4 to 0-1 from the 68th minute onwards but just couldn't fight back to parity in an exciting finale.
Scorers for Clare: D Tubridy 1-2 (0-2f), J Malone, E Cleary (0-3f) 0-3 each, C O'Connor, G Brennan 0-2 each, G Cooney 0-1.
Scorers for Westmeath: K Martin 0-6 (0-3f), T McDaniel 0-3 (0-2f), C McCormack 0-2, R O'Toole, G Egan, T Molloy, N Mulligan (0-1 45) 0-1 each.
Clare: S Ryan; D Ryan, C Brennan, G Kelly; S Collins, A Fitzgerald, C O hAinifein; S O'Donoghue, C O'Connor; E Cleary, G Brennan, J Malone; G Cooney, D Tubridy, C O'Dea.
Subs: K Malone for Cooney (48), E Collins for O hAinineain (51), G O'Brien for O'Connor (62, black card), E O'Connor for C Brennan (70, black card), A Sweeney for S Collins (71).
Westmeath: E Carberry; K Daly, J Smith B Sayeh; R Wallace, K Maguire, J Dolan; J Heslin, D Corroon; D Lynch, G Egan, K Martin; R O'Toole, J Halligan, C McCormack.
Subs: TMcDaniel for Heslin (7, black card), S Duncan for Egan (16), N Mulligan for O'Toole (23, black card), S Pettit for Lynch (58), T Molloy for McDaniel (67), C Slevin for Halligan (68).
Referee: M McNally (Monaghan).