Bill's opponents (4)
I believe the answer is:
noes
'bill's opponents' is the definition.
'noes' can be an answer for 'opponents' (I've seen this before). I am not sure about the 'bill's' bit.
'bill's opponents' is the wordplay.
'bill's' becomes 'no' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'opponents' becomes 'es' (I've seen this in other clues).
'no'+'es'='NOES'
Can you help me to learn more?
(Other definitions for noes that I've seen before include "denials" , "they are dissenting" , "more than one refusal" , "Opponents" , "Votes against a motion" .)