On guard in conflict with Yankee (4)
I believe the answer is:
wary
'on guard' is the definition.
(similar in meaning)
'conflict with yankee' is the wordplay.
'conflict' becomes 'war' (war is a kind of conflict).
'with' is a charade indicator (letters next to each other).
'yankee' becomes 'Y' (phonetic alphabet: alpha, bravo, charlie etc.).
'war'+'y'='WARY'
'in' is the link.
(Other definitions for wary that I've seen before include "Displaying caution" , "James Bond villain" , "Cautious - vigilant" , "Guarded" , "Distrustful" .)