Abhorrent ruling to imprison reversed before magistrates (9)
I believe the answer is:
repugnant
'abhorrent' is the definition.
(synonyms)
'ruling to imprison reversed before magistrates' is the wordplay.
'ruling' becomes 'regnant' (as in a queen regnant, a queen who rules by birth).
'to imprison' is an insertion indicator.
'reversed' is a reversal indicator.
'before magistrates' becomes 'up' (as in being up before a judge).
'up' in reverse letter order is 'pu'.
'regnant' enclosing 'pu' is 'REPUGNANT'.
(Other definitions for repugnant that I've seen before include "Loathsome" , "Abhorrent" , "Offensive to the mind" , "vile" , "Very unwelcome" .)