Discharge during altercation is light (3,4,2)
I believe the answer is:
set fire to
'is light' is the definition.
(thesaurus)
'discharge during altercation' is the wordplay.
'discharge' becomes 'fire' ('fire' can be a synonym of 'discharge'**).
'during' is an insertion indicator.
'altercation' becomes 'setto' (I've seen this before).
'fire' placed into 'setto' is 'SET FIRE TO'.
(Other definitions for set fire to that I've seen before include "Ignite" , "torch" , "Light" .)