As well as a London orchestra (4)
I believe the answer is:
also
'as well as' is the definition.
(I've seen this before)
'a london orchestra' is the wordplay.
'london orchestra' becomes 'LSO' (london symphony orchestra).
'a'+'lso'='ALSO'
(Other definitions for also that I've seen before include "Old" , "To boot" , "What's more" , "Likewise" , "revolutionary a part for Yugoslavia" .)