Cinderella's sisters were unfair (4)

I believe the answer is:

ugly

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

'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.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.