Snigger (6)
I believe the answer is:
titter
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">snigger</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for titter that I've seen before include "Stifled laugh" , "Silly little laugh" , "A high-pitched nervous laugh" , "Nervously restrained laugh" , "Chuckle" .)