Brewed ideal beer -- gas is foul (12)
I believe the answer is:
disagreeable
'is foul' is the definition.
(thesaurus)
'brewed ideal beer gas' is the wordplay.
'brewed' indicates an anagram.
'ideal'+'beer'+'gas'='idealbeergas'
'idealbeergas' anagrammed gives 'DISAGREEABLE'.
(Other definitions for disagreeable that I've seen before include "Bad-tempered" , "Unpleasant" , "Obnoxious" , "Offensive - peevish" .)