Those put on it may expect firm offers in writing (7,4)
I believe the answer is:
mailing list
'writing' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'those put on it may expect firm offers in' is the wordplay.
I cannot quite see how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is located in the answer.
'in' is within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for mailing list that I've seen before include "that'll show where posts should go" , "people selected for posts" .)