Pusher's chaser (4)
I believe the answer is:
narc
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">pusher's chaser</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for narc that I've seen before include "Agent dealing with Charlie" , "Drugs agent (US)" , "member of drug squad" , "One on drugs squad" , "Drugs officer" .)