Horizontal beam (6)
(Other definitions for transom that I've seen before include "Lintel" , "Boat's flat stern - horizontal beam" , "Cross-member of boat or window" , "bolstering member" , "Crossbar over door" .)
'horizontal beam' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for lintel that I've seen before include "Supporting beam over a doorway" , "Horizontal beam over a doorway" , "Supporter" , "It's found above the door" , "Supporting member" .)