Order always set up after month (6)
I believe the answer is:
decree
'order' is the definition.
(I know that order can be written as decree)
'always set up after month' is the wordplay.
'always' becomes 'eer' (poetic way of saying ever).
'set up' says the letters should be written backwards (in a down clue, letters go up).
'after' means one lot of letters go next to another.
'month' becomes 'dec' (abbreviation for December).
'eer' reversed gives 'ree'.
'ree' put after 'dec' is 'DECREE'.
(Other definitions for decree that I've seen before include "Recede (anag.)" , "Edict; court order" , "An official proclamation or ruling" , "Judgment of a court" , "Commandment" .)