Seen in vehicle, keep-fit lark (5)
I believe the answer is:
caper
'lark' is the definition.
(thesaurus)
'seen in vehicle keep-fit' is the wordplay.
'seen in' indicates putting letters inside.
'vehicle' becomes 'car' (car is a kind of vehicle).
'keep fit' becomes 'pe' (I can't explain this - if you can you should give a lot more credence to this answer).
'car' enclosing 'pe' is 'CAPER'.
Can you help me to learn more?
(Other definitions for caper that I've seen before include "Lark" , "Gambol" , "Skip/dance about" , "Pickled flower bud or prank" , "Skip or dance; escapade" .)