An unfair comparison (2,4,2,3)

I believe the answer is:

as ugly as sin

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

'an unfair' is the definition.
I don't know anything about this answer so I can't judge whether this works.

'comparison' is the wordplay.
I cannot really see how this works, but
'comparison' could be 'uglyassin' (I have seen 'Unfair comparison' mean 'ugly as sin' so perhaps 'comparison' could also mean 'ugly as sin') and 'uglyassin' is found in the answer.
The remaining letters 'as' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

I've seen this clue in The Irish News, The Telegraph and The Guardian.

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