Offensive lout hangs about (9)
I believe the answer is:
onslaught
'offensive' is the definition.
(thesaurus)
'lout hangs about' is the wordplay.
'about' indicates anagramming the letters.
'lout'+'hangs'='louthangs'
'louthangs' with letters rearranged gives 'ONSLAUGHT'.
(Other definitions for onslaught that I've seen before include "Violent attack" , "Destructive attack" , "Aggressive action" , "Hunt goals (anag) - offensive" , "Assault" .)