Exceptionally large lead weight mostly comprising certain metal (6)

I believe the answer is:

osmous

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'certain metal' is the definition.
The definition suggests an adjective which matches the answer.

'exceptionally large lead weight mostly' is the wordplay.
I cannot really see how this works, but
'large' could be 'os' (abbreviation for oversize) and 'os' is found in the answer.
The remaining letters 'mous' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).

'comprising' is the link.

Can you help me to learn more?

I've seen this clue in The Times.

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