Litigator (9)
I believe the answer is:
plaintiff
'litigator' is the definition.
(I know that plaintiff is a type of litigator)
This is all the clue.
(Other definitions for plaintiff that I've seen before include "The one taking the court action" , "One bringing a lawsuit" , "one prompting action" , "Temporary disappearances of a heavenly body" , "one in suit" .)