Crossword Genius

What you've done when you're finished (3,2)

Ross

I believe the answer is:

had it

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

'you're finished' is the definition.
I don't know anything about this answer so I can't judge whether this works.

'what you've done' is the wordplay.
I cannot really understand how this works, but
'done' could be 'had' ('do' and 'have' can both mean to defraud) and 'had' is located in the answer.
The remaining letters 'it' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

'when' acts as a link.

Can you help me to learn more?

I've seen this clue in the Evening Standard.
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