Package's back after month on order (6)
I believe the answer is:
decree
'order' is the definition.
(synonyms)
'package's back after month on' is the wordplay.
'back' suggests the final letters (the letter at the back).
'after' means one lot of letters go next to another.
'month' becomes 'dec' (abbreviation for December).
'on' becomes 're' ('on' can be mean 'with reference to').
The final letter of 'package' is 'e'.
'dec'+'re'='decre'
'e' after 'decre' is 'DECREE'.
(Other definitions for decree that I've seen before include "Rule" , "Ordinance" , "Direct" , "Edict; court order" , "Official edict" .)