Sort of deficiency that brings back bitterness in European Union (6)
I believe the answer is:
ullage
'sort of deficiency' is the definition.
(amount by which a vessel falls short of being full)
'back bitterness in european union' is the wordplay.
'back' shows that the letters should be reversed in order.
'bitterness' becomes 'gall' (synonyms).
'in' indicates putting letters inside.
'european union' becomes 'eu'.
'gall' placed within 'eu' is 'egallu'.
'egallu' reversed gives 'ULLAGE'.
'that brings' acts as a link.
(Other definitions for ullage that I've seen before include "dregs" , "The amount by which a bottle falls short of being full" , "Loss by evaporation or leakage" , "Amount by which a liquid container falls short of being full" , "shortfall" .)