Announce when cleared for take off (7)

I believe the answer is:

declare

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'announce' is the definition.
(synonyms)

'cleared for take off' is the wordplay.
'cleared' anagrams to 'DECLARE'.
However, I am unsure how this is indicated.

'when' acts as a link.

Can you help me to learn more?

(Other definitions for declare that I've seen before include "say" , "Announce formally" , "Proclaim" , "Make clearly known" , "State clearly, announce officially" .)

I've seen this clue in the Sydney Morning Herald.

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