Crossword Genius

Is hot? No, perishing, truthfully (7)

Ross

I believe the answer is:

insooth

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'perishing truthfully' is the definition.
I know nothing about this answer so I cannot tell whether this works.

'is hot? no' is the wordplay.
'is hot?' becomes 'in' (both can mean fashionable).
'no' becomes 'sooth' (I can't justify this - if you can you should give a lot more credence to this answer).
'in'+'sooth'='INSOOTH'

Can you help me to learn more?

(Another definition for insooth that I've seen is " Truly (archaic)".)

I've seen this clue in The Times.
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