Humour is about right for a legal document (4)
I believe the answer is:
writ
'a legal document' is the definition.
(writ is a kind of legal document)
'humour is about right' is the wordplay.
'humour' becomes 'wit' ('wit' can be a synonym of 'humour').
'is about' is an insertion indicator.
'right' becomes 'r' (common abbreviation).
'wit' enclosing 'r' is 'WRIT'.
'for' acts as a link.
(Other definitions for writ that I've seen before include "Legal injunction" , "Legal document (usually issued)" , "Court instrument" , "Legal document issued by an officer of the court" , "Legal order in writing" .)