Weight unit for gems (5)
I believe the answer is:
carat
'weight unit for gems' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for carat that I've seen before include "A unit of weight for precious stones" , "Measure for diamonds or gold" , "Jewellery unit" , "Unit of weight for gems or purity of gold" , "Unit of weight for diamonds" .)