An old-fashioned American heater (3)

I believe the answer is:

gat

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'heater' is the definition.
(I've seen this in another clue)

I don't understand how the rest of the clue works.

(Other definitions for gat that I've seen before include "Piece" , "weapon" , "Gangster's gun" , "in Indian music, the last bit of raga" , "Slang for a gangster's pistol" .)

I've seen this clue in the Evening Standard.

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