Uses bananas with instant dessert (6)
I believe the answer is:
mousse
'dessert' is the definition.
(mousse is a kind of dessert)
'uses bananas with instant' is the wordplay.
'bananas' indicates an anagram (informal term for mad).
'with' means one lot of letters goes inside another.
'instant' becomes 'mo' (**).
'uses' is an anagram of 'usse'.
'usse' placed around 'mo' is 'MOUSSE'.
(Other definitions for mousse that I've seen before include "Shock treatment" , "Creamy dish" , "F" , "cold dish" , "Light savoury dish" .)