Good sense shown retaining original of Russian legal document (4)
I believe the answer is:
writ
'legal document' is the definition.
(I know that writ is a type of legal document)
'good sense shown retaining original of russian' is the wordplay.
'good sense' becomes 'wit' (wit can mean sense).
'shown retaining' is an insertion indicator.
'original of' suggests taking the first letters (I've seen 'original' mean this).
The initial letter of 'russian' is 'r'.
'wit' placed around 'r' is 'WRIT'.
(Other definitions for writ that I've seen before include "Legal document of command" , "Summons to Parliament" , "Formal command of law court requiring certain action" , "Legal document (usually issued)" , "Subpoena" .)