Frantically organises to contain end of spring hostility (10)
I believe the answer is:
aggression
'hostility' is the definition.
(synonyms)
'frantically organises to contain end of spring' is the wordplay.
'frantically' indicates an anagram (I've seen this in other clues).
'to contain' is an insertion indicator.
'end of' says to take the final letters.
The last letter of 'spring' is 'g'.
'organises' with letters rearranged gives 'agression'.
'agression' going around 'g' is 'AGGRESSION'.
(Other definitions for aggression that I've seen before include "Act of aggression,physical force" , "What eleven Ac displays" , "Unprovoked attack" , "Act of hostility" , "Belligerence -hostility" .)