Saw to test vehicle in front (5)
I believe the answer is:
motto
'saw' is the definition.
(saw can mean a proverb or saying)
'to test vehicle in front' is the wordplay.
I cannot really understand how this works, but
'to' is present in the answer.
'test' could be 'mot' (as in an MOT test for a vehicle) and 'mot' is found in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for motto that I've seen before include "Words often found on coat of arms" , "statement of belief" , "Recurring musical phrase" , "Cracker slip" , "Slogan encapsulating self-image" .)