Informer or spy (4)
I believe the answer is:
nark
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">informer</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that nark is a type of informer)</span><br/ ><br/ >'<span class="explanation-format__original">spy</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(Other definitions for nark that I've seen before include "Informer - irritate" , "Get to" , "Police spy" , "expert" , "Keep an eye" .)