Endlessly jeered relative's musical performance (10)
I believe the answer is:
hootenanny
'musical performance' is the definition.
Both the definition and answer are singular nouns.
Maybe you can see a link between them that I don't see?
'endlessly jeered relative's' is the wordplay.
'endlessly' means to remove the last letter.
'jeered' becomes 'hooted' (I've seen this in another clue).
'relative's' becomes 'nanny' (I can't justify this - if you can you should give a lot more credence to this answer).
'hooted' with its final letter taken off is 'hoote'.
'hoote'+'nanny'='HOOTENANNY'
Can you help me to learn more?
(Other definitions for hootenanny that I've seen before include "Party involving folk music" , "folk concert" , "Dance" , "Informal folk-singing performance" , "Gathering where folk music gets played" .)