Vehicle made to carry a weight (5)
I believe the answer is:
carat
'weight' is the definition.
(I've seen this before)
'vehicle made to carry a' is the wordplay.
'vehicle' becomes 'cart' (cart is a kind of vehicle).
'made to carry' means one lot of letters goes inside another (some letters carried within others).
'cart' going around 'a' is 'CARAT'.
(Other definitions for carat that I've seen before include "Unit of measurement for the proportion of gold in an alloy" , "weight of rocks" , "Unit of weight of gemstones" , "Measure of weight of diamonds" , "Unit of weight for diamonds" .)