Weight of gems assembled at the front in wagon (5)
I believe the answer is:
carat
'weight of gems' is the definition.
(unit of weight of gemstones)
'assembled at the front in wagon' is the wordplay.
'at the front' says to take the initial letters.
'in' means one lot of letters goes inside another.
'wagon' becomes 'cart' (cart is a kind of wagon).
The first letter of 'assembled' is 'a'.
'a' put within 'cart' is 'CARAT'.
(Other definitions for carat that I've seen before include "Unit of weight used for gems" , "Jewellery unit" , "Measure of fineness of gold" , "Unit of weight for gems and pearls" , "Diamond weight unit" .)