Average beer upset enthusiast (5,6)
I believe the answer is:
eager beaver
'enthusiast' is the definition.
(I've seen this in another clue)
'average beer upset' is the wordplay.
'upset' indicates anagramming the letters.
'average'+'beer'='averagebeer'
'averagebeer' with letters rearranged gives 'EAGER BEAVER'.
(Other definitions for eager beaver that I've seen before include "Very keen individual" , "Enthusiast - zealous person - one too eager to work" , "keen customer" , "Keen worker" , "Diligent worker" .)