Reason behind daft vehicle (10)
I believe the answer is:
locomotive
'vehicle' is the definition.
(locomotive is a kind of vehicle)
'reason behind daft' is the wordplay.
'reason' becomes 'motive' (I've seen this before).
'behind' means one lot of letters go next to another.
'daft' becomes 'loco' (synonyms).
'motive' after 'loco' is 'LOCOMOTIVE'.
(Other definitions for locomotive that I've seen before include "Railway engine" , "for moving from place to place" , "Able to move by oneself" , "Rocket, perhaps" , "one's attached to the train" .)