Not quite a group study centre (3)
I believe the answer is:
uni
'study centre' is the definition.
(university)
'not quite a group' is the wordplay.
'not quite' means to remove the last letter (most but not all of the word).
'a group' becomes 'unit' (eg a unit of soldiers).
'unit' with its final letter taken away is 'UNI'.
(Other definitions for uni that I've seen before include "Queen's, Belfast, for example" , "Short for university" , "Where you can study" , "Third-level college, in short" , "Seat of learning" .)