Observes attack from the South (5)
I believe the answer is:
notes
'observes' is the definition.
('note' can be a synonym of 'observe')
'attack from the south' is the wordplay.
'attack' becomes 'set on' (synonyms).
'from the south' says the letters should be written backwards (in a down clue, letters go up).
'seton' in reverse letter order is 'NOTES'.
(Other definitions for notes that I've seen before include "Lines" , "Pays attention" , "Stone with jottings" , "Cash" , "Memoranda" .)