Threaten battle to the north (4)
I believe the answer is:
warn
'threaten' is the definition.
(I know that threaten is a more specific form of the action the warn)
'battle to the north' is the wordplay.
'battle' becomes 'war' (I've seen this in other clues).
'to the north' becomes 'N' (abbreviation).
'war'+'n'='WARN'
(Other definitions for warn that I've seen before include "Tip off" , "Make aware of danger" , "Alert" , "Advise" , "Admonish, caution" .)