After a month ring: strange, but it's etiquette (7)
I believe the answer is:
decorum
'it's etiquette' is the definition.
(I've seen this before)
'after a month ring strange' is the wordplay.
'after' is a charade indicator (letters next to each other).
'a month' becomes 'Dec' (abbreviation for December).
'ring' becomes 'O' (letter resembling a ring).
'strange' becomes 'rum' (similar in meaning).
'dec'+'o' is 'deco'.
'deco'+'rum'='DECORUM'
'but' acts as a link.
(Other definitions for decorum that I've seen before include "Good manners" , "Proper form" , "Seemliness" , "Decency" , "Correctness or dignity of behaviour" .)