Mass attack on staff letter carrier (7)
I believe the answer is:
mailman
'letter carrier' is the definition.
(I know that letter carrier can be written as mailman)
'mass attack on staff' is the wordplay.
'mass attack' becomes 'mail' (I am not sure about this - if you are sure you should believe this answer much more).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'staff' becomes 'man' (as in staffing or manning a particular place).
'mail'+'man'='MAILMAN'
Can you help me to learn more?
(Other definitions for mailman that I've seen before include "He delivers the post in the US" , "American man of letters" .)