Doorway side post (4)
I believe the answer is:
jamb
'doorway side post' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for jamb that I've seen before include "Upright of door" , "Vertical part to a doorframe" , "Vertical member of door frame" , "Vertical side of a doorway" , "Door sidepiece" .)