Understanding European complaint (5)
I believe the answer is:
gripe
'complaint' is the definition.
(an objection or source of annoyance)
'understanding european' is the wordplay.
'understanding' becomes 'grip' (I've seen this in another clue).
'european' becomes 'E' (abbreviation e.g. EU).
'grip'+'e'='GRIPE'
(Other definitions for gripe that I've seen before include "carp" , "beef" , "Complain, grouse" , "Complain; stomach pain" , "Complain; colic (water)" .)