Intelligence to grasp king's decree (4)
I believe the answer is:
writ
'decree' is the definition.
(I've seen this in another clue)
'intelligence to grasp king's' is the wordplay.
'intelligence' becomes 'wit' (wit is a kind of intelligence).
'to grasp' means one lot of letters goes inside another.
'king' becomes 'R' (abbreviation of Latin rex).
'wit' enclosing 'r' is 'WRIT'.
(Other definitions for writ that I've seen before include "court paper" , "Legal injunction" , "Legal or formal document" , "Legal document issued by an officer of the court" , "Warrant, court order" .)