Deal with a letter as a diplomatic document (6)
I believe the answer is:
treaty
'a diplomatic document' is the definition.
'treaty' can be an answer for 'document' (treaty is a kind of document). I am not sure about the 'a diplomatic' bit.
'deal with a letter' is the wordplay.
'deal with' becomes 'treat' (synonyms).
'a letter' becomes 'y' (Y is a letter of the alphabet).
'treat'+'y'='TREATY'
'as' acts as a link.
Can you help me to learn more?
(Other definitions for treaty that I've seen before include "something of Rome or Versailles?" , "Ratified agreement between states" , "Formal agreement between states" , "Compact - agreement" , "Formal agreement between governments" .)