Vigilant fighting Yankee (4)
I believe the answer is:
wary
'vigilant' is the definition.
(synonyms)
'fighting yankee' is the wordplay.
'fighting' becomes 'war'.
'yankee' becomes 'Y' (phonetic alphabet: alpha, bravo, charlie etc.).
'war'+'y'='WARY'
(Other definitions for wary that I've seen before include "Guarded" , "Cautious - vigilant" , "Cautious of danger" , "Watchful" , "James Bond villain" .)