Nuisance, assassin on medication (10)
I believe the answer is:
painkiller
'medication' is the definition.
(I know that painkiller is a type of medication)
'nuisance assassin on' is the wordplay.
'nuisance' becomes 'pain' (synonyms).
'assassin' becomes 'killer' (hired killer).
'on' means one lot of letters go next to another.
'pain'+'killer'='PAINKILLER'
(Other definitions for painkiller that I've seen before include "Aspirin, for instance" , "Analgesic drug" , "Aspirin maybe" , "soothing drug" , "form of medicine" .)