Act litigiously (3)
I believe the answer is:
sue
'act litigiously' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sue that I've seen before include "Prosecute; girl's name" , "Susan (dim.)" , "Start proceedings against (someone)" , "Take legal case against" , "Take legal action against (someone), girl's name" .)