Propriety after month nothing strange (7)
I believe the answer is:
decorum
'propriety' is the definition.
(I know that decorum is a type of propriety)
'after month nothing strange' is the wordplay.
'after' means one lot of letters go next to another.
'month' becomes 'dec' (abbreviation for December).
'nothing' becomes 'O' (looks like zero - 0).
'strange' becomes 'rum' (synonyms).
'dec' put next to 'o' is 'deco'.
'deco'+'rum'='DECORUM'
(Other definitions for decorum that I've seen before include "modesty" , "(Of outward appearances) what is proper" , "Correctness or dignity of behaviour" , "Proper form" , "Good manners" .)