Obscene ruling covering half of pubs (9)
I believe the answer is:
repugnant
'obscene' is the definition.
('repugnant' can be a synonym of 'obscene')
'ruling covering half of pubs' is the wordplay.
'ruling' becomes 'regnant' ('regnant' can be a synonym of 'ruling'**).
'covering' indicates putting letters inside.
'half of' suggests halving the letters.
'pubs' cut in half is 'pu'.
'regnant' placed around 'pu' is 'REPUGNANT'.
(Other definitions for repugnant that I've seen before include "revolting" , "Very unwelcome" , "Offensive to the mind" , "Disgusting" , "Unacceptable" .)