One being horrible to Bill (4)
I believe the answer is:
toad
'one being horrible' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'to bill' is the wordplay.
'bill' becomes 'ad' (advertisement stuck on wall).
'to'+'ad'='TOAD'
Can you help me to learn more?
(Other definitions for toad that I've seen before include "A loathsome person" , "not found in Ireland" , "Tailless amphibian" , "Amphibian with warts" , "Warty creature" .)