Carriage 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)
'carriage contains a' is the wordplay.
'carriage' becomes 'cart' ().
'contains' is an insertion indicator.
'cart' going around 'a' is 'CARAT'.
(Other definitions for carat that I've seen before include "Measure for diamonds or gold" , "Unit of weight of gemstones" , "Measure of purity of gold" , "Not a large weight" , "Unit of weight for gems and pearls" .)