Some heat, with one major slice of fortune, reversed kidney stones (7)

I believe the answer is:

calculi

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

'stones' is the definition.
(calculus is a kind of stone)

'some heat with one major slice of fortune reversed kidney' is the wordplay.
I cannot really understand how this works, but
'some' could be 'a' and 'a' is located in the answer.
'one' could be 'li' (I've seen this in other clues) and 'li' is located in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Another definition for calculi that I've seen is " gallstones?".)

I've seen this clue in The Telegraph.

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