Unacceptable ruling about backing up (9)
I believe the answer is:
repugnant
'unacceptable' is the definition.
(repugnant means unacceptable or disgusting)
'ruling about backing up' is the wordplay.
'ruling' becomes 'regnant' (as in a queen regnant, a queen who rules by birth).
'about' indicates putting letters inside.
'backing' says the letters should be written in reverse.
'up' reversed gives 'pu'.
'regnant' going around 'pu' is 'REPUGNANT'.
(Other definitions for repugnant that I've seen before include "Abhorrent" , "Extremely distasteful" , "revolting" , "Nasty" , "Loathsome" .)