Police spy or informer (4)
I believe the answer is:
nark
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">police spy</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >'<span class="explanation-format__original">informer</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I know that nark is a type of informer)</span><br/ ><br/ ></span>
(Other definitions for nark that I've seen before include "a nuisance down under" , "Bother" , "cause irritation" , "Annoy; informer (sl.)" , "expert" .)