Be stuck with second class post (4)
I believe the answer is:
'post' is the definition.
(I've seen this before)
'be stuck with second class' is the wordplay.
'be stuck' becomes 'jam' (jam can mean to get stuck).
'with' means one lot of letters go next to another.
'second class' becomes 'b'.
(Other definitions for jamb that I've seen before include "Vertical member of door frame" , "Vertical part to a doorframe" , "part of doorway?" , "Door sidepiece" , "Vertical side of a doorway" .)