Caution about hostilities to the north (4)
I believe the answer is:
warn
'caution' is the definition.
(I've seen this before)
'hostilities to the north' is the wordplay.
'hostilities' becomes 'war' (I've seen this before).
'to the north' becomes 'n' ('N' can be a synonym of 'north').
'war'+'n'='WARN'
'about' acts as a link.
(Other definitions for warn that I've seen before include "Give a caution to" , "Make aware of danger" , "Notify of potential risk" , "Sound the alarm" , "Alert" .)