Criminal ran brothel, mostly obscene (9)
I believe the answer is:
abhorrent
'obscene' is the definition.
(synonyms)
'criminal ran brothel mostly' is the wordplay.
'criminal' indicates an anagram.
'mostly' means to remove the last letter.
'ran'+'brothel'='ranbrothel'
'ranbrothel' with its final letter taken off is 'ranbrothe'.
'ranbrothe' anagrammed gives 'ABHORRENT'.
(Other definitions for abhorrent that I've seen before include "Horrible" , "Repugnant, detestable" , "Disgusting, loathsome" , "execrable" , "hateful" .)