Noiseless (6)
I believe the answer is:
silent
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">noiseless</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 silent that I've seen before include "Unheard" , "Mute" , "Taciturn" , "Quiet -- 13 (anag)" , "Noiseless" .)