Inactive spy (7)
I believe the answer is:
sleeper
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">inactive spy</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 sleeper that I've seen before include "Support for railway lines" , "Form of transport" , "One who's out" , "latent spy" , "One who slumbers" .)