Fighting lion say devouring unruly mob (6)
I believe the answer is:
combat
'fighting' is the definition.
(synonyms)
'lion say devouring unruly mob' is the wordplay.
'lion say' becomes 'cat' (lions are big cats).
'devouring' is an insertion indicator.
'unruly' indicates an anagram.
'mob' with letters rearranged gives 'omb'.
'cat' enclosing 'omb' is 'COMBAT'.
(Other definitions for combat that I've seen before include "Battle, warfare" , "A fight or contest, sometimes mortal" , "Fight or contest, may be mortal" , "Conflict" , "War; contest" .)