Bully proposers horribly (9)
I believe the answer is:
oppressor
'bully' is the definition.
(thesaurus)
'proposers horribly' is the wordplay.
'horribly' indicates anagramming the letters (a horribly wrong version of the letters).
'proposers' with letters rearranged gives 'OPPRESSOR'.
(Other definitions for oppressor that I've seen before include "Tyrant" , "One tyrannises" , "One subjects others to undue pressure" , "Persecutor of proposers" , "Proposers for one who persecutes" .)