Bananas used with an indulgent treat (6)
I believe the answer is:
sundae
'indulgent treat' is the definition.
(type of ice cream)
'bananas used with an' is the wordplay.
'bananas' is an anagram indicator (informal term for mad).
'with' is a charade indicator (letters next to each other).
'used' after 'an' is 'anused'.
'anused' anagrammed gives 'SUNDAE'.
(Other definitions for sundae that I've seen before include "Type of ice-cream" , "Ice-cream/nut etc. dish" , "Ice cream with toppings" , "Ice cream served with a topping" , "Ice-cream dessert" .)