Subscribers don't start on coals (6)

I believe the answer is:

embers

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'coals' is the definition.
('ember' can be a synonym of 'coal')

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

(Other definitions for embers that I've seen before include "Small pieces of glowing wood or coal" , "Vestiges of fire" , "Ashes" , "Hot fragments of wood or coal" , "Live coals" .)

I've seen this clue in The Mirror.

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