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