Part of orchestra once it's formed (7)
I believe the answer is:
section
'part of orchestra' is the definition.
('section' is part of 'orchestra')
'once it's formed' is the wordplay.
'formed' is an anagram indicator.
'once'+'its'='onceits'
'onceits' is an anagram of 'SECTION'.
(Other definitions for section that I've seen before include "branch" , "Cut" , "Subdivision of a newspaper" , "Notices (anag)" , "unit" .)