Test only three-fifths of the car? (3)
I believe the answer is:
mot
'test' is the definition.
(MOT test)
'only three-fifths of the car?' is the wordplay.
'only three fifths of' becomes 'mo' (I can't explain this - if you can you should give a lot more credence to this answer).
'the car?' becomes 't'.
'mo'+'t'='MOT'
Can you help me to learn more?
(Other definitions for mot that I've seen before include "Vehicle check" , "witty remark" , "Word in France, bon perhaps" , "Irish girlfriend" , "Witty saying" .)