Course designers ultimately admitted to waste (7)
I believe the answer is:
Here is my best explanation:
'course' is the definition.
(dessert is a kind of course)
'designers ultimately admitted to waste' is the subsidiary indication.
'designers ultimately' becomes 's' (I can't justify this - if you can you should give a lot more credence to this answer).
'admitted to' means one lot of letters goes inside another.
'waste' becomes 'desert' (a desert may be described as a wasteland).
's' put inside 'desert' is 'dessert'.
(Other definitions for dessert that I've seen before include "Afters", "Pudding, sweet", "Course", "Last course, sweet", "Sweet, last course of a meal" and "Sweet, afters".)