Sibilant summons (4)
I believe the answer is:
psst
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">sibilant summons</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for psst that I've seen before include "(Whispered) may I have your attention?" , "Sound used to attract someone's attention" , "Noise used to attract someone's attention privately" , "Call to attract attention" , "Sibilant sound to get someone's attention" .)