Something implanted inside group (5)
I believe the answer is:
inset
'something implanted' is the definition.
(something set in something else)
'inside group' is the wordplay.
'inside' becomes 'in'.
'group' becomes 'set' (both can mean a collection).
'in'+'set'='INSET'
(Other definitions for inset that I've seen before include "Placed inside" , "Something put into something else" , "Additional insert" , "Photographic cameo superimposed on to another print" , "Piece let into a garment" .)