Decree by leaders of Oceania requires drinking alcohol in November (6)
I believe the answer is:
ordain
'decree' is the definition.
(I know that ordain is a more specific form of the action decree)
'leaders of oceania requires drinking alcohol in november' is the wordplay.
'leaders of' indicates taking the first letters.
'november' becomes 'N' (phonetic alphabet: alpha, bravo, charlie etc.).
The first letters of 'oceania requires drinking alcohol in' is 'ordai'.
'ordai'+'n'='ORDAIN'
'by' is the link.
(Other definitions for ordain that I've seen before include "Officially order" , "Decree authoritatively" , "Decree or appoint to priesthood" , "Lay down (law)" , "Appoint as priest" .)