Weight of vehicle? Answer: tons (5)
I believe the answer is:
carat
'weight' is the definition.
(I've seen this in another clue)
'vehicle? answer tons' is the wordplay.
'vehicle?' becomes 'car' (car is a kind of vehicle).
'answer' becomes 'a' (abbreviation eg in Q and A).
'tons' becomes 't'.
'car'+'a'+'t'='CARAT'
'of' acts as a link.
(Other definitions for carat that I've seen before include "Jewellery unit" , "Precious stone weight" , "Gemstone weight unit" , "Unit of weight for gems or purity of gold" , "Measure of fineness of gold" .)