How gran upset Ann (3)

I believe the answer is:

nan

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'how gran' is the definition.
I can't judge whether this defines the answer.

'upset ann' is the wordplay.
'upset' indicates an anagram.
'ann' anagrammed gives 'NAN'.

Can you help me to learn more?

(Other definitions for nan that I've seen before include "Ancestor" , "Darling dog" , "Grandmother (colloq.)" , "Type of Indian bread" , "Gran" .)

I've seen this clue in the Evening Standard.

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