When you do, are you going, going, gone? (3,8)

I believe the answer is:

bid farewell

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

'when you do are you' is the definition.
I can't judge whether this defines the answer.

'going going gone?' is the wordplay.
I cannot quite understand how this works, but
'going' could be 'farewell' (farewell is a kind of going) and 'farewell' is located in the answer.
The remaining letters 'bid' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in the Evening Standard.

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