Notice month in good grades (8,6)
I believe the answer is:
marching orders
'notice' is the definition.
(I know that marching orders is a type of dismissal)
'month in good grades' is the wordplay.
'month in good' becomes 'marching' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'grades' becomes 'orders' ('order' can be a synonym of 'grade').
'marching'+'orders'='MARCHING ORDERS'
Can you help me to learn more?
(Other definitions for marching orders that I've seen before include "may be issued on the parade ground" , "Notice" , "what's given to departing soldiers?" , "They send you packing" , "given when firing?" .)