Playing piano, Mary shows heated obsession (9)

I believe the answer is:

pyromania

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'heated obsession' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

'playing piano mary' is the wordplay.
'playing' indicates an anagram.
'piano'+'mary'='pianomary'
'pianomary' is an anagram of 'PYROMANIA'.

'shows' is the link.

Can you help me to learn more?

(Other definitions for pyromania that I've seen before include "Fire-raising impulse" , "Criminal urge" , "Obsessive interest in fire" , "igniting lots of fires" , "Pay Marion (anag.)" .)

I've seen this clue in The Daily Express.

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