Sponge in a dessert rising (5)
I believe the answer is:
loofa
'sponge' is the definition.
(I've seen this before)
'a dessert rising' is the wordplay.
'dessert' becomes 'fool' (type of dessert).
'rising' is a reversal indicator (in a down clue, letters go up).
'a'+'fool'='afool'
'afool' in reverse letter order is 'LOOFA'.
'in' acts as a link.
(Other definitions for loofa that I've seen before include "bathing accessory" , "Bathroom sponge" , "clean-up with this" , "American scrubber" , "Fruit used for a sponge" .)