Drug setter took after surgery (6)
I believe the answer is:
opiate
'drug' is the definition.
(I know that opiate is a type of narcotic)
'setter took after surgery' is the wordplay.
'setter' becomes 'I' (the person that set the crossword).
'took' becomes 'ate' (eating is a kind of taking).
'after' means one lot of letters go next to another.
'surgery' becomes 'op' (short for operation).
'i'+'ate'='iate'
'iate' put after 'op' is 'OPIATE'.
(Other definitions for opiate that I've seen before include "description of religion?" , "A sedative drug" , "of heroin, perhaps" , "Poppy-based drug" , "A poet I become on this drug" .)