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Cinderella's sisters were unfair (4)

Ross

I believe the answer is:

ugly

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'unfair' is the definition.
Both the answer and definition are adjectives. Maybe they are linked in a way I don't understand?

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for ugly that I've seen before include "'Unattractive, unsightly (4)'" , "Lacking in beauty" , "Displeasing to the senses" , "No oil painting!" , "Disquieting" .)

I've seen this clue in The Globe and Mail.
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