German beer upset by bird (5)
I believe the answer is:
grebe
'bird' is the definition.
(type of bird)
'german beer upset' is the wordplay.
'german' becomes 'g' (abbreviation).
'upset' is an anagram indicator.
'beer' is an anagram of 'rebe'.
'g'+'rebe'='GREBE'
'by' acts as a link.
(Other definitions for grebe that I've seen before include "Duck-sized water-bird" , "Small diving waterfowl with a long neck" , "Diver" , "Small bird that builds floating nests" , "Diving waterbird" .)