Fix top of bedroom doorpost (4)
I believe the answer is:
jamb
'doorpost' is the definition.
(thesaurus)
'fix top of bedroom' is the wordplay.
'fix' becomes 'jam' ('jam' can be a synonym of 'fix').
'top of' indicates taking the first letters.
The initial letter of 'bedroom' is 'b'.
'jam'+'b'='JAMB'
(Other definitions for jamb that I've seen before include "Upright sidepiece" , "Vertical side of a doorway" , "Side post of a doorway" , "Side of door frame" , "Door, window side post" .)