Measure back door (4)
I believe the answer is:
rood
'measure' is the definition.
(thesaurus)
'back door' is the wordplay.
'back' says the letters should be written backwards.
'door' back-to-front is 'ROOD'.
(Other definitions for rood that I've seen before include "Old land measure and large cross in a church" , "Crucifix on part of an acre" , "Crucifix; old land area measure" , "old piece of land" , "- Loft; - screen" .)