Tactful notes in official container (10,3)
I believe the answer is:
diplomatic bag
'official container' is the definition.
I know nothing about this answer so I can't tell whether this works.
'tactful notes' is the wordplay.
'tactful' becomes 'diplomatic' (similar in meaning).
'notes' becomes 'bag' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'diplomatic'+'bag'='DIPLOMATIC BAG'
'in' is the link.
Can you help me to learn more?
(Other definitions for diplomatic bag that I've seen before include "Embassy mail sack" , "One taking post abroad" , "Embassy's mail container" , "Embassy mail pouch" .)