Quite simple disagreement but he goes to court (9)
I believe the answer is:
plaintiff
'he goes to court' is the definition.
(I've seen this before)
'quite simple disagreement' is the wordplay.
'quite simple' becomes 'plain' (similar in meaning. I am not sure about the 'quite' bit.).
'disagreement' becomes 'tiff' (tiff is a kind of disagreement).
'plain'+'tiff'='PLAINTIFF'
'but' is the link.
Can you help me to learn more?
(Other definitions for plaintiff that I've seen before include "accusing person in suit" , "The litigant making the accusation" , "One bringing civil court case" , "one in suit" , "he goes to court" .)