Once the area of a back door? (4)
I believe the answer is:
rood
'once the area' is the definition.
I can't tell whether this definition defines the answer.
'a back door?' is the wordplay.
'a back' is a reversal indicator.
'door' back-to-front is 'ROOD'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for rood that I've seen before include "Pole, perch or crucifix" , "Cross, crucifix" , "Crucifix - old unit of area" , "Crucifix in entrance, set back" , "Old land measure and large cross in a church" .)