Small cake presented with good sweetener (4)
I believe the answer is:
bung
'sweetener' is the definition.
(both can mean a bribe)
'small cake presented with good' is the wordplay.
'small cake' becomes 'bun' (a bun is a small cake).
'presented with' says to put letters next to each other.
'good' becomes 'g' (abbreviation).
'bun'+'g'='BUNG'
(Other definitions for bung that I've seen before include "Drum; antelope" , "Cask stopper" , "illegal incentive" , "Tip, bribe (sl.)" , "Spigot or cork" .)