Nightmare (7)
I believe the answer is:
incubus
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">nightmare</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for incubus that I've seen before include "Burdensome person or evil spirit visiting sleeper" , "Evil spirit visiting a sleeping person" , "Betraying Apostle" , "Demon bringing nightmares" , "Male demon - nightmare - obsession" .)