Decree from old duke stuck in wet weather (6)
I believe the answer is:
ordain
'decree' is the definition.
(ordaining is a kind of decreeing)
'old duke stuck in wet weather' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'duke' becomes 'd'.
'stuck in' indicates putting letters inside.
'wet weather' becomes 'rain' (I've seen this before).
'd' placed within 'rain' is 'rdain'.
'o'+'rdain'='ORDAIN'
'from' acts as a link.
(Other definitions for ordain that I've seen before include "Lay down (law)" , "Initiate as priest" , "Make someone a clergyman" , "Consecrate as priest" , "Make a decree" .)