Doctor taken aback 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 taken aback with german article' is the wordplay.
'doctor' becomes 'doc' (synonyms).
'taken aback' is a reversal indicator.
'with' means one lot of letters go next to another.
'german article' becomes 'eine' (indefinite article in German).
'doc' in reverse letter order is 'cod'.
'cod'+'eine'='CODEINE'
'on' acts as a link.
(Other definitions for codeine that I've seen before include "An anodyne" , "Mild narcotic used as a pain-killer" , "Painkiller derived from opium" , "Pain-relieving derivative of opium" , "Common opium-based tablet used to relieve pain" .)