Weight of a cart, possibly? (5)
I believe the answer is:
carat
'weight' is the definition.
(I've seen this in another clue)
'a cart possibly?' is the wordplay.
'possibly?' is an anagram indicator (I've seen 'possibly' mean this).
'a'+'cart'='acart'
'acart' with letters rearranged gives 'CARAT'.
'of' acts as a link.
(Other definitions for carat that I've seen before include "Unit of gem measurement" , "Metal weight" , "Unit of weight for diamonds" , "Unit of weight for gems or purity of gold" , "A unit of weight for precious stones" .)