Unnoticeable (13)
I believe the answer is:
imperceptible
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">unnoticeable</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 imperceptible that I've seen before include "Too small to be noticed" , "Unnoticeable" , "Impossible to feel" , "Barely detectable" , "Too slight to be noticed" .)