Letters received (7)
(Other definitions for mailbag that I've seen before include "Sent down sewer's product" , "Postal sack" , "Sack for post" , "Container for carrying letters" , "One sewn by convicts" .)
'letters received' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for postbag that I've seen before include "What one receives collectively" , "one's letters" , "Letters received" , "carrier for Royal Mail" , "All delivered" .)