Gold box Rubbra emptied for ensemble (9)
I believe the answer is:
orchestra
'for ensemble' is the definition.
(thesaurus)
'gold box rubbra emptied' is the wordplay.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'box' becomes 'chest' (chest is a kind of box).
'emptied' says to hollow out the word (remove centre letters).
'rubbra' with its middle removed is 'ra'.
'or'+'chest'+'ra'='ORCHESTRA'
(Other definitions for orchestra that I've seen before include "LSO, for example" , "Large band of musicians" , "Brass, woodwind, strings and percussion" , "Group of musicians found on cart-horse" , "those who provide the music" .)