An orchestra's on top of that (4)
I believe the answer is:
also
'on top of that' is the definition.
('on top of that' means also or in addition)
'an orchestra's' is the wordplay.
'an' becomes 'a'.
'orchestra' becomes 'LSO' (London Symphony Orchestra).
'a'+'lso'='ALSO'
(Other definitions for also that I've seen before include "in accompaniment" , "revolutionary a part for Yugoslavia" , "item's rarely used now for this" , "Additionally" , "Item" .)