Drug operation I polished off (6)
I believe the answer is:
opiate
'drug' is the definition.
(opiate is a kind of drug)
'operation i polished off' is the wordplay.
'operation' becomes 'op'.
'polished off' becomes 'ate' (I've seen this in other clues).
'op'+'i'+'ate'='OPIATE'
(Other definitions for opiate that I've seen before include "Narcotic drug" , "A sleep-inducing drug (6)" , "drug to aid rest" , "of heroin, perhaps" , "Drug from poppies" .)