After work one ingested drug (6)
I believe the answer is:
opiate
'drug' is the definition.
(opiate is a kind of drug)
'after work one ingested' is the wordplay.
'after' means one lot of letters go next to another.
'work' becomes 'op' (abbreviation for opus).
'one' becomes 'i' (Roman numeral).
'ingested' becomes 'ate' (eating is a kind of ingesting).
'op' put next to 'i' is 'opi'.
'opi'+'ate'='OPIATE'
(Other definitions for opiate that I've seen before include "Narcotic, such as morphine" , "Poppy-derived drug" , "Does it ease pain" , "A sedative drug" , "Narcotic drug" .)