Arrivals bearing a lot of mail for writers (9)
I believe the answer is:
composers
'writers' is the definition.
(eg people who write music)
'arrivals bearing a lot of mail' is the wordplay.
'arrivals' becomes 'comers' (people who have arrived).
'bearing' is an insertion indicator.
'a lot of' means to remove the last letter (I've seen 'lots of' mean this (the majority of the word)).
'mail' becomes 'post' (synonyms).
'post' with its final letter taken away is 'pos'.
'comers' going around 'pos' is 'COMPOSERS'.
'for' is the link.
(Other definitions for composers that I've seen before include "Mozart, Puccini" , "Chopin, Strauss etc." .)