Carriage includes 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 includes a' is the wordplay.
'carriage' becomes 'cart'.
'includes' is an insertion indicator.
'cart' going around 'a' is 'CARAT'.
(Other definitions for carat that I've seen before include "Unit of weight for diamonds" , "unit employed among rocks" , "Unit of weight for gems or purity of gold" , "Unit for precious stone weights" , "Metal weight" .)