Beam over a doorway (6)
I believe the answer is:
lintel
'beam over a doorway' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for lintel that I've seen before include "It's found above the door" , "Horizontal timber support" , "Support across doorway" , "Horizontal beam over a door or window" , "Part of door or window frame" .)