Wow, group stays part of this? (6)
I believe the answer is:
corset
'stays part of this?' is the definition.
(stays meaning boned undergarment)
'wow group' is the wordplay.
'wow' becomes 'cor' (both are expressions of surprise).
'group' becomes 'set' (both can mean a collection).
'cor'+'set'='CORSET'
(Other definitions for corset that I've seen before include "Tight-fitting undergarment" , "Close-fitting foundation garment" , "Tight undergarment" , "Sector (anag)" , "Tight-fitting bodice" .)