Doctor upset with German article on painkiller (7)
I believe the answer is:
codeine
'painkiller' is the definition.
(I know that codeine is a type of painkiller)
'doctor upset with german article' is the wordplay.
'doctor' becomes 'doc' (synonyms).
'upset' says the letters should be written in reverse.
'with' means one lot of letters go next to another.
'german article' becomes 'eine' (indefinite article in German).
'doc' written backwards gives 'cod'.
'cod'+'eine'='CODEINE'
'on' acts as a link.
(Other definitions for codeine that I've seen before include "Pain-relieving derivative of opium" , "Common painkiller" , "Derivative of opium used as an analgesic" , "Analgesic drug" , "Niece, do take this drug" .)