After operation, I swallowed a painkiller (6)
I believe the answer is:
opiate
'a painkiller' is the definition.
(I've seen this before)
'after operation i swallowed' is the wordplay.
'after' is a charade indicator (letters next to each other).
'operation' becomes 'op'.
'swallowed' becomes 'ate'.
'op'+'i' is 'opi'.
'opi'+'ate'='OPIATE'
(Other definitions for opiate that I've seen before include "It induces sleep" , "Poppy-derived drug" , "A sleep-inducing drug (6)" , "drug to aid rest" , "Preparation made from poppy derivative" .)