Slit in the back of a jacket (4)
I believe the answer is:
vent
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">slit in the back of a jacket</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 vent that I've seen before include "Voice" , "Airhole" , "Discharge; give expression to" , "Outlet; slit in jacket" , "Opening; release (fluid)" .)