He goes to court for a simple quarrel (9)
I believe the answer is:
plaintiff
'he goes to court' is the definition.
(I've seen this before)
'simple quarrel' is the wordplay.
'simple' becomes 'plain' (similar in meaning).
'quarrel' becomes 'tiff' (informal term for an argument).
'plain'+'tiff'='PLAINTIFF'
'for a' acts as a link.
(Other definitions for plaintiff that I've seen before include "Person bringing a civil action in a court of law" , "One bringing legal case" , "Temporary disappearances of a heavenly body" , "One bringing civil court case" , "One bringing a lawsuit" .)