Killjoy's standard mistake: unfinished musical work (5,6)
I believe the answer is:
party pooper
'killjoy's' is the definition.
(synonyms)
'standard mistake unfinished musical work' is the wordplay.
'standard' becomes 'par' (standard of play in golf).
'mistake' becomes 'typo' (spelling mistake).
'unfinished' means to remove the last letter.
'musical work' becomes 'opera' (operas are musical works).
'opera' with its last letter taken off is 'oper'.
'par'+'typo'+'oper'='PARTY-POOPER'
(Other definitions for party pooper that I've seen before include "Spoilsport" , "One who spoils a social occasion" , "One spoiling others' fun" , "Social 1 across" .)