Posting message, initially poorly (7)
I believe the answer is:
mailing
'posting' is the definition.
(I know that posting can be written as mailing)
'message initially poorly' is the wordplay.
'initially' suggests taking the first letters.
'poorly' becomes 'ailing' (synonyms).
The first letter of 'message' is 'm'.
'm'+'ailing'='MAILING'
(Other definitions for mailing that I've seen before include "Sending a message" , "Dispatch by post" , "Posting" , "Sending by post" .)