Legal document comedian must keep right (4)
I believe the answer is:
writ
'legal document' is the definition.
(I know that writ is a type of legal document)
'comedian must keep right' is the wordplay.
'comedian' becomes 'wit' (a witty person).
'must keep' indicates putting letters inside.
'right' becomes 'r' (common abbreviation).
'wit' going around 'r' is 'WRIT'.
(Other definitions for writ that I've seen before include "Sealed document" , "It may be served" , "Warrant, court order" , "Legal document of command" , "Legal document, usually served" .)