A bribe where you don't want anything to leak out (4)

I believe the answer is:

bung

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'a bribe' is the definition.
(I've seen this before)

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

(Other definitions for bung that I've seen before include "Stopper, plug" , "Stopper for closing hole in barrel" , "Somehow stop" , "sweetener" , "Rubber stopper" .)

I've seen this clue in the Evening Standard.

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