This chap a rock, might we presume? (7,6)

I believe the answer is:

diamond geezer

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'a rock might we presume?' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.

'this chap' is the wordplay.
I cannot quite understand how this works, but
'chap' could be 'geezer' (both can mean a man) and 'geezer' is located in the answer.
The remaining letters 'diamond' 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 Times.

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