Old monarch at first just omits his name (4)
I believe the answer is:
john
'old monarch' is the definition.
(King John, King of England 1199-1216)
'first just omits his name' is the wordplay.
'first' says to take the initial letters.
The initial letters of 'just omits his name' is 'JOHN'.
'at' acts as a link.
(Other definitions for john that I've seen before include "and 24: A Beatle" , "Fourth gospel" , "Book" , "Man's name" , "toilet" .)