Backhander to prevent a leak? (4)

I believe the answer is:

bung

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'leak?' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

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

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(Other definitions for bung that I've seen before include "sweetener" , "Large cork" , "Rubber stopper" , "Spigot or cork" , "Bribe - bottle stopper" .)

I've seen this clue in the Evening Standard.

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