Good form last month nothing unusual (7)
I believe the answer is:
decorum
'good form' is the definition.
(I've seen this before)
'last month nothing unusual' is the wordplay.
'last month' becomes 'dec' (short for December, last month of the year).
'nothing' becomes 'O' (looks like zero - 0).
'unusual' becomes 'rum' (I've seen this in other clues).
'dec'+'o'+'rum'='DECORUM'
(Other definitions for decorum that I've seen before include "This is becoming" , "Decency" , "Proper behaviour; etiquette" , "good behaviour" , "Protocol" .)