Prospective MP needs one constituent in division (9)
I believe the answer is:
nominator
I believe this clue is a double definition.
'prospective mp needs one' is the first definition.
'constituent in division' is the second definition.
(alternative term for numerator)
(Other definitions for nominator that I've seen before include "One proposing candidate" , "someone who makes proposal" , "The proposer" , "One who puts forward a candidate for election" , "One who proposes" .)