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' means one lot of letters goes inside another.
'cart' enclosing '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" .)