Essence of cake that's rising (3)
I believe the answer is:
nub
'essence' is the definition.
(I know that essence can be written as nub)
'cake that's rising' is the wordplay.
'cake' becomes 'bun' (bun is a type of cake).
'that's rising' says the letters should be written backwards (in a down clue, letters go up).
'bun' reversed gives 'NUB'.
'of' acts as a link.
(Other definitions for nub that I've seen before include "Crux, gist" , "Crux, central point" , "Central point of issue" , "Point or gist" , "Gist - kernel" .)