Unit of heat (5)

I think it could be either:

Which length description are you looking for?:

therm

'unit of heat' is the definition.
(I've seen this in another clue)

This is the entire clue.

(Other definitions for therm that I've seen before include "Indicator of heat" , "Unit of heat and gas measure" , "Unit of heat energy" , "Unit of measurement of heat" , "Amount of heat" .)

I've seen this clue in The Mirror, The Independent, The Guardian, The Daily Mail, The Times and The Telegraph.

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