Don't notice that I have added to the letter (7)
I believe the answer is:
missive
'letter' is the definition.
(synonyms)
'don't notice that i have added to' is the wordplay.
'don't notice' becomes 'miss' (I can't explain this - if you can you should believe this answer much more).
'that i have' becomes 'Ive' (contraction of 'I have').
'added to' is a charade indicator (letters next to each other).
'miss'+'ive'='MISSIVE'
'the' is the link.
Can you help me to learn more?
(Other definitions for missive that I've seen before include "message" , "Letter" , "intelligence in this?" .)