Criminal organises, embracing start of gangster hostility (10)
I believe the answer is:
aggression
'hostility' is the definition.
(I've seen this before)
'criminal organises embracing start of gangster' is the wordplay.
'criminal' indicates an anagram.
'embracing' is an insertion indicator.
'start of' indicates taking the first letters.
The first letter of 'gangster' is 'g'.
'organises' with letters rearranged gives 'agression'.
'agression' going around 'g' is 'AGGRESSION'.
(Other definitions for aggression that I've seen before include "Hostile action" , "Belligerence -hostility" , "Use of armed force" , "warlike attitude" , "What eleven Ac displays" .)