Downed one German beer when drunk (5)
I believe the answer is:
grebe
'downed one' is the definition.
'german beer when drunk' is the wordplay.
'german' becomes 'g' (abbreviation).
'when drunk' indicates anagramming the letters (a drunken or disorderly form of the letters).
'beer' is an anagram of 'rebe'.
'g'+'rebe'='GREBE'
(Other definitions for grebe that I've seen before include "Flier" , "Small bird that builds floating nests" , "Small diving waterfowl with a long neck" , "Duck-sized water-bird" , "Swimmer" .)