Warned of an altered possibility (7)

I believe the answer is:

alerted

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'warned' is the definition.
(alerting is a kind of warning)

'an altered possibility' is the wordplay.
'altered' anagrams to 'ALERTED'.
Though, I'm unsure how the anagram is indicated.

'of' is the link.

Can you help me to learn more?

(Other definitions for alerted that I've seen before include "Put on guard, warned" , "Put on guard, forewarned" , "Given signal" , "Tipped off" , "Related about warning notified" .)

I've seen this clue in the Evening Standard.

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