Argument, something that's unpacked in lawsuit (4)
I believe the answer is:
case
I believe this clue is a triple definition!
'argument' is the first definition.
(as in making one's case)
'something that's unpacked' is the second definition.
(a case might be unpacked)
'in' acts as a link.
'lawsuit' is the third definition.
(a legal case)
(Other definitions for case that I've seen before include "Cover" , "A proceeding in a court of law" , "Carefully inspect < suit" , "proceedings" , "Patient - luggage" .)