Alternatively, row about a decree (6)
I believe the answer is:
ordain
'decree' is the definition.
(ordaining is a kind of decreeing)
'alternatively row about a' is the wordplay.
'alternatively' becomes 'or'.
'row' becomes 'din' (both can mean a loud disturbance).
'about' means one lot of letters goes inside another.
'din' enclosing 'a' is 'dain'.
'or'+'dain'='ORDAIN'
(Other definitions for ordain that I've seen before include "frock" , "Decree or appoint to priesthood" , "Decree and admit to holy orders" , "Initiate as priest" , "Make someone a clergyman" .)