American orchestra in London too (4)
I believe the answer is:
also
'too' is the definition.
(synonyms)
'american orchestra' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'orchestra' becomes 'LSO' (London Symphony Orchestra).
'a'+'lso'='ALSO'
'in london' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for also that I've seen before include "an indication of more to come" , "Item" , "To boot" , "in accompaniment" , "'Besides, moreover (4)'" .)