Dessert in diet unusually generous (13)
I believe the answer is:
disinterested
'generous' is the definition.
(thesaurus)
'dessert in diet unusually' is the wordplay.
'unusually' indicates anagramming the letters.
'dessert'+'in'+'diet'='dessertindiet'
'dessertindiet' with letters rearranged gives 'DISINTERESTED'.
(Other definitions for disinterested that I've seen before include "Desire dentist (anag) - free from partiality" , "Impartial" , "Not expecting any profit" , "Objective - uninvolved" , "Feeling no concern" .)