Slit in the back of a jacket (4)
I believe the answer is:
vent
'slit in the back of a jacket' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for vent that I've seen before include "Voice" , "Airhole" , "Discharge; give expression to" , "Outlet; slit in jacket" , "Opening; release (fluid)" .)