Obviously not a plaintive part-song (4)
I believe the answer is:
glee
'part-song' is the definition.
(I know that glee is a type of partsong)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for glee that I've seen before include "Great joy" , "much fun" , "Unholy mirth or triumphant joy" , "Elation" , "Great pleasure" .)