Holly cut off singular killjoy (5)
I believe the answer is:
sprig
'holly cut off' is the definition.
'singular killjoy' is the wordplay.
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'killjoy' becomes 'prig'.
's'+'prig'='SPRIG'
(Other definitions for sprig that I've seen before include "Small nail without a head" , "Purloined" , "Grips with a small branch" , "Spray or twig" , "What may be flowering" .)