Something added is a letter dictated (6)
I believe the answer is:
annexe
'something added' is the definition.
I can't judge whether this defines the answer.
'a letter dictated' is the wordplay.
'a' becomes 'an'.
'letter' becomes 'n'.
'dictated' becomes 'exe' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'an'+'n'+'exe'='ANNEXE'
'is' is the link.
Can you help me to learn more?
(Other definitions for annexe that I've seen before include "Joined-on building" , "An addition to a document or building" , "Extra accommodation block" , "bit on the side?" , "Extension, supplement" .)