To keep down right, ruler has to oust Marxist leader (7)
I believe the answer is:
repress
'to keep down' is the definition.
(synonyms)
'right ruler has to oust marxist leader' is the wordplay.
I cannot quite understand how this works, but
'right' could be 'r' (common abbreviation) and 'r' is found in the answer.
'ruler' could be 'r' (abbreviation for rex, king in Latin) and 'r' is found in the answer.
'leader' could be 'e' (I've seen this in other clues) and 'e' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for repress that I've seen before include "Curb" , "Subdue" , "bottle up" , "Keep in subjection" , "Restrict freedom of" .)