Car test providing theme (5)
I believe the answer is:
motif
'theme' is the definition.
(motif is a kind of theme)
'car test providing' is the wordplay.
'car test' becomes 'mot'.
'providing' becomes 'if' (I've seen this in other clues).
'mot'+'if'='MOTIF'
(Other definitions for motif that I've seen before include "recurring idea" , "Recurrent theme in music..." , "Recurring theme in musical work" , "Theme or melody" , "Shape" .)