Plug the gap in s_eptic? (4)

I believe the answer is:

cork

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'plug' is the definition.
(I know that cork is a type of plug)

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

(Other definitions for cork that I've seen before include "stop" , "Irish county - bottle stopper" , "Stopper - Irish port" , "One stops" , "Buoyant substance from a tree" .)

I've seen this clue in The Independent.

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