Group within a group (6)
I believe the answer is:
subset
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">group within a group</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)