Flash chaps in test of vehicle (6)
I believe the answer is:
moment
'flash' is the definition.
(I've seen this before)
'chaps in test of vehicle' is the wordplay.
'chaps' becomes 'men' (chap can mean a man).
'in' means one lot of letters goes inside another.
'test of vehicle' becomes 'MOT'.
'men' put inside 'mot' is 'MOMENT'.
(Other definitions for moment that I've seen before include "force to turn things around" , "Instant - importance" , "Significance; minute" , "Second, importance" , "Importance - point in time" .)