Crossword Genius

Ours is sour (7)

Ross

I believe the answer is:

anagram

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'sour' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for anagram that I've seen before include "Word rearranging another's letters" , "Word(s) rearranged to form others" , "A ragman (anag)" , "Hedonism for Sondheim" , "Nag a ram (anag)" .)

I've seen this clue in the Evening Standard.
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