Collies be terribly hostile (9)
I believe the answer is:
bellicose
'hostile' is the definition.
(thesaurus)
'collies be terribly' is the wordplay.
'terribly' indicates an anagram.
'collies'+'be'='colliesbe'
'colliesbe' with letters rearranged gives 'BELLICOSE'.
(Other definitions for bellicose that I've seen before include "Aggressive and willing to fight" , "Warlike" , "wanting fight" , "Jingoistic" , "Belligerent" .)