Crucifix put up in entrance (4)
I believe the answer is:
door
'entrance' is the definition.
(I've seen this before)
'crucifix put up' is the wordplay.
'crucifix' becomes 'rood' ('rood' can be a synonym of 'crucifix').
'put up' says the letters should be written backwards.
'rood' backwards is 'DOOR'.
'in' acts as a link.
(Other definitions for door that I've seen before include "Alternative to knocker" , "Threshold" , "Means of exit and entrance" , "Portal" , "See 1" .)