A warning that's sounded later perhaps (5)

I believe the answer is:

alert

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

'a warning' is the definition.
(I know that alert is a type of warning)

'sounded later perhaps' is the wordplay.
'later' anagrams to 'ALERT'.
But, I'm not clear how this is indicated.

'that's' acts as a link.

Can you help me to learn more?

(Other definitions for alert that I've seen before include "Quick to notice" , "Watchful, sharp" , "Warning signal" , "On one's toes" , "Signal danger" .)

I've seen this clue in The Globe and Mail.

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