Decree, one in Jordan judge disregarded (6)
I believe the answer is:
ordain
'decree' is the definition.
(ordaining is a kind of decreeing)
'one in jordan judge disregarded' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' means one lot of letters goes inside another.
'judge' becomes 'j' (abbreviation for judge).
'disregarded' indicates named letters should be taken away.
'i' going within 'jordan' is 'jordain'.
'jordain' with 'j' removed is 'ORDAIN'.
(Other definitions for ordain that I've seen before include "Elevate or appoint to priesthood" , "Inroad (anag.)" , "Install as priest" , "Rule" , "Officially order" .)