Actor knocking back some tonic, apparently (6)
I believe the answer is:
pacino
'actor' is the definition.
(I've seen this before)
'knocking back some tonic apparently' is the wordplay.
'knocking back' says the letters should be written in reverse.
'some' indicates a hidden word (some parts of the word are used).
'onicap' is hidden in the letters of 'tonic apparently'.
'onicap' backwards is 'PACINO'.
(Other definitions for pacino that I've seen before include "Al --, actor" , "Hollywood star" .)