Spoilsport (7)
'spoilsport' is the definition.
(I know that spoilsport can be written as killjoy)
This is all the clue.
(Other definitions for killjoy that I've seen before include "Misery" , "Wet blanket" , "Party pooper" , "One can't cheer up" , "Puritan" .)
(Other definitions for downer that I've seen before include "Informally a depressant drug" , "A sad thing" , "Saddening experience" , "Sedative" , "Depressing experience" .)