Those who mail advertisements (7)
I believe the answer is:
posters
'those' is the definition.
I can't tell whether this defines the answer.
'mail advertisements' is the wordplay.
'mail' becomes 'post' (synonyms).
'advertisements' becomes 'ers' (I can't explain this - if you can you should give a lot more credence to this answer).
'post'+'ers'='POSTERS'
'who' is the link.
Can you help me to learn more?
(Other definitions for posters that I've seen before include "Placards, notices" , "Bills or placards on walls" , "Contributors to forum" , "Public placards" .)