Former occurrence during cool days (3)

I believe the answer is:

old

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'former' is the definition.
(both can mean previous)

'during cool days' is the wordplay.
'during' says the answer is hidden in the clue.
'OLD' is hidden in the letters of 'cool days'.

'occurrence' is the link.
This may not be right. It may be part of another bit of the clue.

Can you help me to learn more?

(Other definitions for old that I've seen before include "SECOND PART OF 14" , "Former, previous" , "Ancient" , "Getting on" , "Elderly" .)

I've seen this clue in the Evening Standard.

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