Choral work is capable of going on so long! (7)
I believe the answer is:
cantata
'choral work' is the definition.
(I've seen this in another clue)
'capable of going on so long' is the wordplay.
'capable of going' becomes 'can' (I can't justify this - if you can you should believe this answer much more).
'on' says to put letters next to each other.
'so long' becomes 'tata' (both are ways of saying goodbye).
'can'+'tata'='CANTATA'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for cantata that I've seen before include "piece of music" , "Work for accompanied voice" , "Short oratorio performed in concert" , "(a musical work)" , "Vocal piece" .)