Not to mention a London symphony orchestra (4)
I believe the answer is:
also
'not to mention' is the definition.
(I've seen this in another clue)
'a london symphony orchestra' is the wordplay.
I cannot really understand how this works, but
'a' is present in the answer.
'orchestra' could be 'lso' (London Symphony Orchestra) and 'lso' is located in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for also that I've seen before include "admitted to Royal Society > too" , "Furthermore" , "To boot" , "item's rarely used now for this" , "'Besides, moreover (4)'" .)