Active way of working on hostility (6)
I believe the answer is:
mobile
'active' is the definition.
(thesaurus)
'way of working on hostility' is the wordplay.
'way of working' becomes 'mo' (abbreviation for modus operandi).
'on hostility' becomes 'bile' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'mo'+'bile'='MOBILE'
Can you help me to learn more?
(Other definitions for mobile that I've seen before include "See 17" , "Movable, phone say" , "American port" , "Able to be moved about - like modern phone" , "Moveable, phone say" .)