Litigate (3)
I believe the answer is:
sue
'litigate' is the definition.
(I know that litigate can be written as sue)
This is all the clue.
(Other definitions for sue that I've seen before include "Little Susan" , "Litigious girl" , "Humbly request" , "Take proceedings against (a person)" , "Bring charges against - girl" .)