Jack of diamonds found under vehicle? (4)
I believe the answer is:
card
'jack' is the definition.
(I've seen this before)
'diamonds found under vehicle?' is the wordplay.
'diamonds' becomes 'd' (abbreviation in bridge say).
'found under' is a charade indicator (letters next to each other).
'vehicle?' becomes 'car' (car is a kind of vehicle).
'd' put after 'car' is 'CARD'.
'of' acts as a link.
(Other definitions for card that I've seen before include "king or queen, for example" , "witty type" , "Amusing person" , "Heart or club, say?" , "Eg, a heart or club" .)