Police spy (4)
I believe the answer is:
nark
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">police spy</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 nark that I've seen before include "Bother" , "Keep an eye" , "Grass; irritate" , "Stool pigeon" , "cause irritation" .)