The setter, after work, swallowed painkiller (6)
I believe the answer is:
opiate
'painkiller' is the definition.
(I've seen this in another clue)
'the setter after work swallowed' is the wordplay.
'the setter' becomes 'I' (the person who set this crossword).
'after' says to put letters next to each other.
'work' becomes 'op' (short for opus).
'swallowed' becomes 'ate' (swallow can mean to eat).
'i' put after 'op' is 'opi'.
'opi'+'ate'='OPIATE'
(Other definitions for opiate that I've seen before include "Sedating drug" , "Drug with an opium base" , "Poppy-derived drug" , "A drug to induce sleep" , "A poet I become on this drug" .)