Secretly follow (6)

I believe the answer is:
shadow
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">secretly follow</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for shadow that I've seen before include "Ghost" , "Unilluminated area" , "This opposition cabinet parallels the government" , "It's cast by something blocking light" , "Partial or complete darkness" .)