Operetta set back across border, culminating with a party (3,6)
I believe the answer is:
the mikado
'operetta' is the definition.
(I've seen this before)
'across border culminating with a party' is the wordplay.
'across' indicates putting letters inside.
'border' becomes 'hem' (I've seen this in another clue).
'culminating' becomes 'tika' (I can't justify this - if you can you should believe this answer much more).
'with' says to put letters next to each other.
'a party' becomes 'do' (do is a kind of party).
'hem' put within 'tika' is 'themika'.
'themika'+'do'='THE MIKADO'
'set back' 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 the mikado that I've seen before include "Staged work" , "G & S operetta" , "Gilbert & Sullivan's Japanese operetta" , "Operetta set in Japan" , "Eighteen eighty five G&S operetta" .)