Not new to being unreasonably sued (4)
I believe the answer is:
used
'not new' is the definition.
(used or pre-owned as opposed to new)
'unreasonably sued' is the wordplay.
'unreasonably' indicates anagramming the letters.
'sued' anagrammed gives 'USED'.
'to being' acts as a link.
(Other definitions for used that I've seen before include "with a previous owner" , "Brought into play" , "Second hand" , "was milking" , "Pre-loved" .)