Spoilsport (7)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">spoilsport</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I know that spoilsport can be written as killjoy)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)