Stick book in the post (4)
I believe the answer is:
jamb
'the post' is the definition.
(I've seen this before)
'stick book' is the wordplay.
'stick' becomes 'jam' (I've seen this in other clues).
'book' becomes 'b' (abbreviation for book).
'jam'+'b'='JAMB'
'in' is the link.
(Other definitions for jamb that I've seen before include "Door sidepiece" , "Doorpost" , "Upright of door" , "Vertical part to a doorframe" , "part of doorway?" .)