Initiate fight very loudly, stopped by knockout (5,3)
I believe the answer is:
spark off
'initiate' is the definition.
(sparking off is a kind of initiating)
'fight very loudly stopped by knockout' is the wordplay.
'fight' becomes 'spar' (sparring is a kind of fighting).
'very loudly' becomes 'ff' (short for fortissimo, 'very loud' in music).
'stopped by' is an insertion indicator (stopping can mean blocking a gap).
'knockout' becomes 'ko' (abbreviation).
'spar'+'ff'='sparff'
'sparff' enclosing 'ko' is 'SPARK OFF'.
(Another definition for spark off that I've seen is " Provide the stimulus for (something)".)