Support section of the orchestra that is backing the Queen (9)
I believe the answer is:
brassiere
'support' is the definition.
(item worn to support breasts)
'section of the orchestra that is backing the queen' is the wordplay.
'section of the orchestra' becomes 'brass' (brass instruments).
'that is' becomes 'ie' (synonyms).
'backing' shows that the letters should be reversed in order.
'the queen' becomes 'er' (Elizabeth Regina).
'er' reversed gives 're'.
'brass'+'ie'+'re'='BRASSIERE'
(Other definitions for brassiere that I've seen before include "garment" , "support" , "Female undergarment supporter" .)