Wagon contains a quantity of diamonds (5)
I believe the answer is:
carat
'quantity of diamonds' is the definition.
(carat is a unit for weighing gemstones)
'wagon contains a' is the wordplay.
'wagon' becomes 'cart' (cart is a kind of wagon).
'contains' means one lot of letters goes inside another.
'cart' going around 'a' is 'CARAT'.
(Other definitions for carat that I've seen before include "Gem weight unit" , "Unit for precious stone weights" , "Metal weight" , "Measure of gold in alloy" , "Unit of measurement for the proportion of gold in an alloy" .)