In favour of ending peace by violence (5)
I believe the answer is:
force
'violence' is the definition.
(synonyms)
'in favour of ending peace' is the wordplay.
'in favour of' becomes 'for' (being for a particular cause).
'ending peace' becomes 'ce' (I am not sure about this - if you are sure you should believe this answer much more).
'for'+'ce'='FORCE'
'by' is the link.
Can you help me to learn more?
(Other definitions for force that I've seen before include "Strength, power" , "Lean on" , "Power; army" , "Impetus" , "cascade of water" .)