Fed up with European Commission? Time to change allegiance (6)
I believe the answer is:
defect
'change allegiance' is the definition.
(I've seen this before)
'fed up with european commission? time' is the wordplay.
'fed up' becomes 'def' ('fed' going up in a down clue).
'with' is a charade indicator (letters next to each other).
'european commission?' becomes 'ec'.
'time' becomes 't' (abbreviation).
'def'+'ec'+'t'='DEFECT'
'to' is the link.
(Other definitions for defect that I've seen before include "go over to the other side" , "Desert - flaw" , "Failing or imperfection" , "Flaw; turn traitor" , "'Flaw, imperfection (6)'" .)