Proposer (5)
I believe the answer is:
mover
'proposer' is the definition.
(synonyms)
This is all the clue.
(Other definitions for mover that I've seen before include "Instigator, organiser" , "One who is in motion - may also be a shaker" , "someone who brings forward proposal" , "Formal proposer" , "Someone who proposes a motion for debate" .)