Coal left out is set alight (6)

I believe the answer is:

ignite

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'set alight' is the definition.
(I've seen this before)

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

(Other definitions for ignite that I've seen before include "Detonate" , "Swan constellation" , "Put a match to" , "Catch fire; provoke" , "Touch off; inflame" .)

I've seen this clue in The Sun and The Times.

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