Assist (a criminal) (4)
I believe the answer is:
abet
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">assist a criminal</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 abet that I've seen before include "Crookedly endorse" , "Incite (to commit an offence)" , "Beat up by way of help" , "Assist in some nefarious activity" , "Aid in offence" .)