Some pumice in the coal? That’s funny! (7)

I believe the answer is:

comical

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'funny' is the definition.
('comical' can be a synonym of 'funny')

'some pumice in the coal?' is the wordplay.
'some pumice' becomes 'mic' (I can't explain this - if you can you should give a lot more credence to this answer).
'in the' is an insertion indicator.
'mic' placed within 'coal' is 'COMICAL'.

'that's' acts as a link.

Can you help me to learn more?

(Other definitions for comical that I've seen before include "'Amusing, funny (7)'" , "Rich" , "Amusing or droll" , "Humorous" , "priceless" .)

I've seen this clue in the Evening Standard.

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