Maestro presenting opera at home in Ulster (9)
I believe the answer is:
toscanini
'maestro' is the definition.
(I've seen this before)
'opera at home in ulster' is the wordplay.
'opera' becomes 'tosca' (I've seen this before**).
'at home' becomes 'in' ('in' can be similar in meaning to 'at home').
'in' means one lot of letters goes inside another.
'ulster' becomes 'ni' (Northern Ireland).
'in' inserted inside 'ni' is 'nini'.
'tosca'+'nini'='TOSCANINI'
'presenting' acts as a link.
(Other definitions for toscanini that I've seen before include "Music director" , "Virtuoso conductor" , "Bare rock formation" , "He conducted" , "Maestro" .)