Order a Yankee to serve (4)
I believe the answer is:
obey
'serve' is the definition.
(thesaurus)
'order a yankee' is the wordplay.
'order' becomes 'obe'.
'a yankee' becomes 'y' (phonetic alphabet: alpha, bravo, charlie etc.).
'obe'+'y'='OBEY'
'to' acts as a link.
(Other definitions for obey that I've seen before include "Do as ordered" , "Perversely inclined to do the opposite" , "Toe the line" , "Do what one is told" , "Act in accordance with (law)" .)