From the start, introduces severely Marxist ideology (3)
I believe the answer is:
ism
'ideology' is the definition.
(I've seen this in another clue)
'from the start introduces severely marxist' is the wordplay.
'from the start' says to take the initial letters.
The initial letters of 'introduces severely marxist' is 'ISM'.
(Other definitions for ism that I've seen before include "Suffix denoting doctrine or practise" , "Ideological movement" , "Informally, any doctrine or practice" , "Suffix denoting doctrine or practice" , "Any system of beliefs" .)