Back entrance? Makes one cross! (4)
I believe the answer is:
rood
'one cross' is the definition.
(rood is a kind of cross)
'back entrance?' is the wordplay.
'back' is a reversal indicator.
'entrance?' becomes 'door' (door is a kind of entrance).
'door' written backwards gives 'ROOD'.
'makes' is the link.
(Other definitions for rood that I've seen before include "Crucifix - old unit of area" , "Area equal to 40 square rods" , "Old land measure of quarter of an acre" , "Cross, crucifix" , "old piece of land" .)