A cosmetic (8)
I believe the answer is:
eyeliner
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">a cosmetic</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 eyeliner that I've seen before include "highlight for look-out" , "it draws attention to the viewer" , "Makeup for the peepers" , "Type of make-up" , "Dark marks left by this" .)