Swept ashes — when left inside, there’s trouble (6)

I believe the answer is:

hassle

'trouble' is the definition.
(I know that trouble can be written as hassle)

I don't understand the rest of the clue.

(Other definitions for hassle that I've seen before include "Difficulty - dispute" , "badger" , "Annoy continually" , "Informal annoyance or quarrel" , "Pester (colloq.)" .)

I've seen this clue in The Guardian.

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