One orchestra, too (4)
I believe the answer is:
also
'too' is the definition.
(synonyms)
'one orchestra' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'orchestra' becomes 'LSO' (London Symphony Orchestra).
'a'+'lso'='ALSO'
(Other definitions for also that I've seen before include "an indication of more to come" , "To boot" , "'Besides, moreover (4)'" , "in accompaniment" , "into the bargain" .)