Plug a bribe (4)
I believe the answer is:
bung
I believe this clue is a double definition.
'plug' is the first definition.
(bung is a kind of plug)
'a bribe' is the second definition.
(I've seen this before)
(Other definitions for bung that I've seen before include "Somehow stop" , "Bribe - bottle stopper" , "Rubber stopper" , "Spigot or cork" , "Large cork" .)