The Leaving - of course, it's nearly past its sell-by-date (5,3)

I believe the answer is:

going off

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'nearly past its sell-by-date' is the definition.
I can't judge whether this defines the answer.

'the leaving of course' is the wordplay.
'the leaving' becomes 'going' ('going' can be a synonym of 'leaving').
'of course' becomes 'off' (I can't justify this - if you can you should believe this answer much more).
'going'+'off'='GOING OFF'

'it's' acts as a link.

Can you help me to learn more?

I've seen this clue in the Irish Times.

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