Carriage contains a quantity of gold (5)
I believe the answer is:
carat
'quantity of gold' is the definition.
(as in 24-carat gold etc)
'carriage contains a' is the wordplay.
'carriage' becomes 'cart'.
'contains' means one lot of letters goes inside another.
'cart' placed around 'a' is 'CARAT'.
(Other definitions for carat that I've seen before include "Metal weight" , "Measure of purity of gold" , "Unit of gem measurement" , "Unit for precious stone weights" , "Unit of measurement for the proportion of gold in an alloy" .)