Hostility among outsiders initially greeting mixed seniors (10)
I believe the answer is:
aggression
'hostility' is the definition.
(I know that hostility can be written as aggression)
'among outsiders initially greeting mixed seniors' is the wordplay.
'outsiders' says to hollow out the word (remove centre letters) (only the letters on the outside of the word remain).
'initially' says to take the initial letters.
'mixed' is an anagram indicator.
'among' with its middle taken out is 'ag'.
The first letter of 'greeting' is 'g'.
'seniors' with letters rearranged gives 'ression'.
'ag'+'g'+'ression'='AGGRESSION'
(Other definitions for aggression that I've seen before include "Belligerence -hostility" , "warlike attitude" , "Violent hostility" , "Act of aggression,physical force" , "Act of hostility" .)