Constituent ingredient of carrot/celery turnover! (7)
I believe the answer is:
elector
'constituent' is the definition.
('elector' can be a synonym of 'constituent')
'ingredient of carrotcelery turnover' is the wordplay.
'ingredient of' indicates a hidden word.
'turnover' is a reversal indicator.
'rotcele' is hidden in the letters of 'carrotcelery'.
'rotcele' backwards is 'ELECTOR'.
(Other definitions for elector that I've seen before include "Someone with the right to vote" , "One whom may vote" , "A prince of the Holy Roman Empire" , "One eligible to vote" , "One entitled to vote" .)