Kept hidden (6)
I believe the answer is:
secret
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">kept hidden</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for secret that I've seen before include "Private, or something private" , "Inner" , "Security classification level" , "Confidence" , "Esoteric" .)