Be an informer (5)
I believe the answer is:
grass
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">be an informer</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 grass that I've seen before include "Shopper" , "Common plant" , "one may be urged to keep off" , "German writer" , "Slang word meaning both marijuana and informant" .)