Entirely Yankee band together (4)
I believe the answer is:
ally
'band together' is the definition.
(I've seen this in another clue)
'entirely yankee' is the wordplay.
'entirely' becomes 'all' (all can mean entirely or completely).
'yankee' becomes 'Y' (phonetic alphabet: alpha, bravo, charlie etc.).
'all'+'y'='ALLY'
(Other definitions for ally that I've seen before include "Join by marriage, treaty etc" , "Team up" , "Fighting partner" , "One in cooperation with another" , "Someone with whom one is in league" .)