Drug I swallowed after operation (6)
I believe the answer is:
opiate
'drug' is the definition.
(opiate is a kind of drug)
'i swallowed after operation' is the wordplay.
'swallowed' becomes 'ate'.
'after' is a charade indicator (letters next to each other).
'operation' becomes 'op'.
'i'+'ate'='iate'
'iate' put after 'op' is 'OPIATE'.
(Other definitions for opiate that I've seen before include "Poppy-based drug" , "Drug from poppies" , "description of religion?" , "Drug inducing a false sense of contentment" , "one soporific" .)