Claimant's apparent argument (9)
I believe the answer is:
plaintiff
'claimant's' is the definition.
(I've seen this before)
'apparent argument' is the wordplay.
'apparent' becomes 'plain' (synonyms).
'argument' becomes 'tiff' (I've seen this in other clues).
'plain'+'tiff'='PLAINTIFF'
(Other definitions for plaintiff that I've seen before include "one in suit" , "Former litigant" , "he goes to court" , "Suitor" , "Person bringing a court case" .)