She upset Carlo at Christmas

I believe the answer is:

carol

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

'she' is the definition.

'upset carlo at christmas' is the wordplay.
'carlo' can be anagrammed to 'CAROL'.
Though, I am unsure how the anagram is indicated.

Can you help me to learn more?

(Other definitions for carol that I've seen before include "Coral (anag.)" , "Joyful religious song" , "Joyful song - girl's name" , "Girl" , "Song, especially at Christmas" .)

I've seen this clue in the Evening Standard.

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