Litigates (4)
I believe the answer is:
sues
'litigates' is the definition.
('sue' can be a synonym of 'litigate')
This is the entire clue.
(Other definitions for sues that I've seen before include "Proceeds legally against" , "Takes legal action against" , "Institutes legal proceedings against" , "Takes to court" , "Takes a case against" .)