Group within a group (6)
I believe the answer is:
subset
'group within a group' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for subset that I've seen before include "small class?" , "Group of which all the elements are contained in a larger group" , "Not all the members" , "Group within a group" , "Small group nested inside larger group" .)