Contradict Yankee on retreat (4)
I believe the answer is:
deny
'contradict' is the definition.
(denying is a kind of contradicting)
'yankee on retreat' is the wordplay.
'yankee' becomes 'Y' (phonetic alphabet: alpha, bravo, charlie etc.).
'on' says to put letters next to each other.
'retreat' becomes 'den'.
'y' put after 'den' is 'DENY'.
(Other definitions for deny that I've seen before include "Repudiate, disavow" , "Declare non-existent" , "Disown" , "Contradict, refuse" , "Gainsay" .)