Changed when alerted? (7)

I believe the answer is:

altered

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

'changed' is the definition.
(I know that changed can be written as altered)

'alerted?' is the wordplay.
'alerted' can be anagrammed to 'ALTERED'.
However, I'm unsure how the anagram is indicated.

'when' acts as a link.

Can you help me to learn more?

(Other definitions for altered that I've seen before include "Reformed" , "Change" , "Changed, modified" , "Metamorphosed" , "Treadle (anag)" .)

I've seen this clue in the Evening Standard.

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