Pack diamonds into vehicles (5)
I believe the answer is:
cards
'pack' is the definition.
(thesaurus)
'diamonds into vehicles' is the wordplay.
'diamonds' becomes 'd' (abbreviation in bridge say).
'into' is an insertion indicator.
'vehicles' becomes 'cars' (car is a kind of vehicle).
'd' going within 'cars' is 'CARDS'.
(Other definitions for cards that I've seen before include "Playing pack" , "They may be for playing or greeting" , "jokers" , "Odd people" , "Christmas post" .)