Beam over door or window (6)
I believe the answer is:
lintel
'beam over door or window' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for lintel that I've seen before include "Horizontal beam over a door or window" , "Support across the top of a door" , "Supporting member" , "Horizontal beam over a doorway" , "Supportive member" .)