Yes, it’s a mere forgery the spy got hold of (9)
I believe the answer is:
agreement
'yes' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Maybe there's a link between them I don't understand?
'a mere forgery the spy got hold of' is the wordplay.
I cannot really understand how this works, but
'a' is found in the answer.
an anagram of 'mere' is 'reem' which is present in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is located in the answer.
The remaining letters 'gen' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for agreement that I've seen before include "understanding" , "Compact, accord" , "Correspondence" , "Harmony of opinion" , "Harmony, concurrence" .)