Gathered and ingested a single drug (9)
I believe the answer is:
methadone
'drug' is the definition.
(methadone is a kind of drug)
'gathered and ingested a single' is the wordplay.
'gathered' becomes 'met' ('meet' can be a synonym of 'gather').
'and' means one lot of letters go next to another.
'ingested' becomes 'had' ('have' can be a synonym of 'ingest').
'a single' becomes 'one' (a single thing - only one).
'met'+'had'+'one'='METHADONE'
(Other definitions for methadone that I've seen before include "Drug used in treating heroin addiction" , "Synthetic drug taken to replace heroin" , "Narcotic analgesic drug" , "Synthetic analgesic" .)