Garment, small, Gershwin brother returned (4)
I believe the answer is:
sari
'garment' is the definition.
(I know that sari is a garment)
'small gershwin brother returned' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'gershwin brother' becomes 'ira' (I've seen this in another clue).
'returned' says the letters should be written backwards.
'ira' written backwards gives 'ari'.
's'+'ari'='SARI'
(Other definitions for sari that I've seen before include "Indian robe" , "Something worn" , "Traditional Indian women's dress" , "Traditional Indian garment" , "Traditional dress of Indian women" .)