Alert to a battle to the east (5)
I believe the answer is:
aware
'alert' is the definition.
('aware' can be a synonym of 'alert')
'a battle to the east' is the wordplay.
'battle' becomes 'war' (I've seen this before).
'to the east' becomes 'e' ('E' can be a synonym of 'east').
'a'+'war'+'e'='AWARE'
'to' is the link.
(Other definitions for aware that I've seen before include "Cognisant, mindful" , "Conscious, sentient" , "Having knowledge or realisation" , "Acquainted with or conscious of" , "Bearing in mind" .)