After work I swallowed drug (6)
I believe the answer is:
opiate
'drug' is the definition.
(opiate is a kind of drug)
'after work i swallowed' is the wordplay.
'after' says to put letters next to each other.
'work' becomes 'op' (abbreviation for opus).
'swallowed' becomes 'ate'.
'op' next to 'i' is 'opi'.
'opi'+'ate'='OPIATE'
(Other definitions for opiate that I've seen before include "Preparation made from poppy derivative" , "Drug from poppies" , "A drug to induce sleep" , "Does it ease pain" , "drug to aid rest" .)