I'm top name in performance: Xmas one! (9)

I believe the answer is:

pantomime

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

'i'm top name in performance' is the wordplay.
'i'm top name' anagrams to 'PANTOMIME'.
However, I'm not sure how the anagram is indicated.

Can you help me to learn more?

(Other definitions for pantomime that I've seen before include "Cross-dressing theatrical" , "Theatrical entertainment (usually at Christmas)" , "Farcical situation" , "'He's behind you' entertainment" , "Traditional Christmas stage show with Dame" .)

I've seen this clue in The Sun.

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