Weight of gems, 100, found with a deserter (5)
I believe the answer is:
carat
'weight of gems' is the definition.
(unit of weight of gemstones)
'100 found with a deserter' is the wordplay.
'100' becomes 'C' (Roman numeral).
'found with' says to put letters next to each other.
'deserter' becomes 'rat' (both can mean a traitor).
'c'+'a'+'rat'='CARAT'
(Other definitions for carat that I've seen before include "weight of rocks" , "Jewellery unit" , "small weight" , "Unit of weight of gemstones" , "Unit for precious stone weights" .)