Unit of heat energy (7)

I think it could be either:

Which length description are you looking for?:

therm

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

This is all the clue.

(Other definitions for therm that I've seen before include "Former gas supply unit" , "Unit of heat used in the UK as the basis of gas bills" , "Indicator of heat" , "Unit of measurement of heat" , "Unit of heat as in gas supply" .)

I've seen this clue in the Sydney Morning Herald.

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