German to knock father for drink (6)
I believe the answer is:
grappa
'for drink' is the definition.
(type of alcoholic drink)
'german to knock father' is the wordplay.
'german' becomes 'g' (abbreviation).
'to knock' becomes 'rap' ('rap' can be a synonym of 'knock').
'father' becomes 'pa' (pa can mean dad or father).
'g'+'rap'+'pa'='GRAPPA'
(Other definitions for grappa that I've seen before include "Italian firewater" , "Brandy made from wine press residue" , "Spirit distilled from fermented grape remains" , "Italian spirit made from grape residue after pressing" , "Type of brandy" .)