Crossword Genius

At whom to shout disapproval never-endingly? (4)

Ross

I believe the answer is:

boor

I'm a little stuck... Click here to teach me more about this clue!

'at' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

'to shout disapproval never-endingly?' is the wordplay.
I cannot really see how this works, but
'disapproval' could be 'boo' (as in a crowd booing) and 'boo' is located in the answer.
A single letter 'r' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

'whom' is the link.

Can you help me to learn more?

(Other definitions for boor that I've seen before include "Coarse, ill-mannered person" , "Lout" , "Rude uncouth person" , "Rude, uncultured type" , "Barbarian" .)

I've seen this clue in the Evening Standard.
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