Jewellery unit caught a despicable person (5)
I believe the answer is:
carat
'jewellery unit' is the definition.
(I've seen this before)
'caught a despicable person' is the wordplay.
'caught' becomes 'c' (cricket abbreviation).
'despicable person' becomes 'rat' (I've seen this before).
'c'+'a'+'rat'='CARAT'
(Other definitions for carat that I've seen before include "Unit of weight for gems or purity of gold" , "Unit of weight for diamonds" , "Measure applied to gold or gems" , "Measure of purity of gold" , "Unit of measurement for the proportion of gold in an alloy" .)