Sweetener found in cake and sugar, essentially (4)
I believe the answer is:
bung
'sweetener' is the definition.
(both can mean a bribe)
'cake and sugar essentially' is the wordplay.
'cake' becomes 'bun' (I've seen this before).
'and' is a charade indicator (letters next to each other).
'essentially' indicates the central letters.
The central letter of 'sugar' is 'g'.
'bun'+'g'='BUNG'
'found in' acts as a link.
(Other definitions for bung that I've seen before include "Throw; stopper" , "Somehow stop" , "Spigot or cork" , "Cask stopper" , "Rubber stopper" .)