Dirty as it can be back room (7)
I believe the answer is:
stained
'dirty' is the definition.
(thesaurus)
'as it can be back room' is the wordplay.
'can be' indicates anagramming the letters.
'back' says the letters should be written backwards.
'room' becomes 'den' (den is a kind of room**).
'den' backwards is 'ned'.
'as'+'it'='asit'
'asit' is an anagram of 'stai'.
'stai'+'ned'='STAINED'
(Other definitions for stained that I've seen before include "Like many church windows" , "Church feature - side sold was wanting (anag)" , "Marked, discoloured" , "Soiled" , "___ glass" .)