Good sense to retain Republican’s legal document (4)
I believe the answer is:
writ
'legal document' is the definition.
(document from a court)
'good sense to retain republican's' is the wordplay.
'good sense' becomes 'wit' (wit can mean sense).
'to retain' is an insertion indicator.
'republican' becomes 'R' (political abbreviation).
'wit' placed around 'r' is 'WRIT'.
(Other definitions for writ that I've seen before include "Legal order in writing" , "Court instrument" , "Legal document issued by an officer of the court" , "Court order document" , "Legal or formal document" .)