Unit caught a traitor (5)
I believe the answer is:
carat
'unit' is the definition.
(carat is a kind of unit)
'caught a traitor' is the wordplay.
'caught' becomes 'c' (cricket abbreviation).
'traitor' becomes 'rat' (I've seen this before).
'c'+'a'+'rat'='CARAT'
(Other definitions for carat that I've seen before include "Measure of gold in alloy" , "Unit of weight for gems or purity of gold" , "Unit for precious stone weights" , "weight of rocks" , "Gem weight unit" .)