Good American group making strengthener for garment (6)
I believe the answer is:
gusset
'strengthener for garment' is the definition.
'good american group' is the wordplay.
'good' becomes 'g' (abbreviation).
'american' becomes 'us'.
'group' becomes 'set' (set is a kind of group).
'g'+'us'+'set'='GUSSET'
'making' acts as a link.
(Other definitions for gusset that I've seen before include "Angular insertion in a garment" , "Strengthening piece of material" , "Strengthening material insert" , "Gets us (anag.)" , "Piece of material set into a dress" .)