Mock, jeer (4)
I believe the answer is:
hiss
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">mock</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ >'<span class="explanation-format__original">jeer</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(Other definitions for hiss that I've seen before include "Catcall" , "Make disapproving sound" , "Sound of serpent" , "Sharp sibilant sound" , "Sibilant sound made through the teeth" .)