Navy on battle alert (4)
I believe the answer is:
warn
'alert' is the definition.
(thesaurus)
'navy on battle' is the wordplay.
'navy' becomes 'n' (common abbreviation eg in RN Royal Navy).
'on' means one lot of letters go next to another.
'battle' becomes 'war' (I've seen this before**).
'n' after 'war' is 'WARN'.
(Other definitions for warn that I've seen before include "Give advance notice to" , "Notify in advance of danger" , "Admonish, caution" , "Advise" , "Tip off" .)