Excellent study group (5)
I believe the answer is:
class
I believe this clue is a double definition.
'excellent' is the first definition.
(I've seen this before)
'study group' is the second definition.
(group of people studying together)
(Other definitions for class that I've seen before include "School form" , "Group - style" , "Set, category" , "Lesson; social division" , "First-rate" .)