Struggle to categorise conflict (5,3)
I believe the answer is:
class war
'conflict' is the definition.
(I know that class war is a type of conflict)
'struggle to categorise' is the wordplay.
'struggle' becomes 'war' (warring is a kind of struggling).
'to' says to put letters next to each other (I've seen this in other clues).
'categorise' becomes 'class' (classing is a kind of categorizing).
'war' put after 'class' is 'CLASS WAR'.
(Other definitions for class war that I've seen before include "Revolutionary activity" , "preached by Lenin?" , "Struggle the proletariat is scheduled to win" , "Conflict in Marxist theory" , "Conflict in society" .)