Fag smoke? (7)

I believe the answer is:

exhaust

This clue is a double definition.

'fag' is the first definition.
(exhausting is a kind of fagging)

'smoke?' is the second definition.
(thesaurus)

(Other definitions for exhaust that I've seen before include "Car's waste gas" , "Waste gas from engine" , "lower part of estate?" , "part of car - > tire?" , "Weary" .)

I've seen this clue in The Telegraph.

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