The previous warning was given prior to leaving (9)

I believe the answer is:

foregoing

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'the previous' is the definition.
(I've seen this before)

'warning was given prior to leaving' is the wordplay.
I cannot really see how this works, but
'warning' could be 'fore' (used to warn of airborne balls on a golf course) and 'fore' is found in the answer.
'leaving' could be 'going' (synonyms) and 'going' is found within the answer.
No letters remain.
This explanation may well be incorrect...

Can you help me to learn more?

(Another definition for foregoing that I've seen is " Previous".)

I've seen this clue in the Evening Standard.

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