Weight unit (5)
I believe the answer is:
ounce
'weight unit' is the definition.
(I know that ounce is a type of troy unit)
This is the entire clue.
(Other definitions for ounce that I've seen before include "Wildcat" , "Jaguar" , "Unit of weight - snow leopard" , "one of a number in the pound" , "Small unit of weight" .)