Crossword Genius

One being horrible to Bill (4)

Ross

I believe the answer is:

toad

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'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" .)

I've seen this clue in The Sun.
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