Cross over a door (4)
I believe the answer is:
rood
'cross' is the definition.
(term for a crucifix)
'over a door' is the wordplay.
'over' is a reversal indicator.
'door' written backwards gives 'ROOD'.
(Other definitions for rood that I've seen before include "Crucifix; old land area measure" , "The Holy . . . . was the Cross of Christ" , "Crucifix in entrance, set back" , "Crucifix on door" , "Crucifix in church; one quarter of an acre" .)