Leave about a quarter of the fruit course (7)
I believe the answer is:
dessert
'course' is the definition.
(I know that dessert is a type of course)
'leave about a quarter of the fruit' is the wordplay.
'leave' becomes 'desert' (desert can mean to leave or abandon).
'about' is an insertion indicator.
'a quarter of the fruit' becomes 's' (I am not sure about this - if you are sure you should believe this answer much more).
'desert' enclosing 's' is 'DESSERT'.
Can you help me to learn more?
(Other definitions for dessert that I've seen before include "Fruit and nuts, perhaps" , "Spotted Dick is one" , "Christmas pudding maybe" , "Fool possibly" , "Sweet, afters" .)