Saw to test on vehicle in front (5)
I believe the answer is:
motto
'saw' is the definition.
(I've seen this before)
'test on vehicle in front' is the wordplay.
'test' becomes 'mot'.
'on' means one lot of letters go next to another.
'vehicle in front' becomes 'to' (I can't justify this - if you can you should believe this answer much more).
'mot'+'to'='MOTTO'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for motto that I've seen before include "Message in cracker" , "Maxim; slogan" , "Saying; slogan" , "Repeated theme in music" , "Saying in a cracker" .)