Follow secretly (6)
I believe the answer is:
shadow
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">follow secretly</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 shadow that I've seen before include "It's cast when light is blocked" , "Follow closely - dark image - opposition" , "Sort of boxer - dog" , "opposing counterpart" , "Follow secretly; obscure" .)