Attacked over messages (5)
I believe the answer is:
notes
'messages' is the definition.
(note is a kind of message)
'attacked over' is the wordplay.
'attacked' becomes 'seton' (I've seen this in other clues).
'over' says the letters should be written backwards.
'seton' backwards is 'NOTES'.
(Other definitions for notes that I've seen before include "Short missives" , "output from musicians" , "Cash" , "Memoranda" , "Memos; marks" .)