Further a London orchestra (4)
I believe the answer is:
also
'further' is the definition.
(I've seen this in another clue)
'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 "Furthermore" , "admitted to Royal Society > too" , "Plus" , "into the bargain" , "in accompaniment" .)