Hostility rose and gang is disbanded (10)
I believe the answer is:
aggression
'hostility' is the definition.
(aggression is a kind of hostility)
'rose and gang is disbanded' is the wordplay.
'and' means one lot of letters go next to another.
'disbanded' is an anagram indicator (I've seen this in other clues).
'gang'+'is'='gangis'
'rose' put after 'gangis' is 'gangisrose'.
'gangisrose' with letters rearranged gives 'AGGRESSION'.
(Other definitions for aggression that I've seen before include "Hostile action" , "Use of armed force" , "Act of aggression,physical force" , "warlike attitude" , "Unprovoked attack" .)