Inner circle in a Venn diagram (6)
I believe the answer is:
subset
'inner circle in a venn diagram' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for subset that I've seen before include "small class?" , "Not all the members" , "A collection of things forming part of a larger collection" , "Small group nested inside larger group" , "Group of which all the elements are contained in a larger group" .)