Gold casket’s real — occasionally, part of it may be brass! (9)
'part of it may be brass' is the definition.
(eg a chest might have brass clasps or handles)
'gold casket's real occasionally' is the wordplay.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'casket' becomes 'chest'.
'occasionally' indicates alternate letters.
The alternating letters of 'real' are 'ra'.
(Other definitions for orchestra that I've seen before include "Noted performers", "bandsmen?", "Ensemble of players", "Music ensemble", "Large band of players".)