Bad end, Yankee to refute (4)
I believe the answer is:
deny
'to refute' is the definition.
(I've seen this in another clue)
'bad end yankee' is the wordplay.
'bad' is an anagram indicator.
'yankee' becomes 'Y' (phonetic alphabet: alpha, bravo, charlie etc.).
'end' anagrammed gives 'den'.
'den'+'y'='DENY'
(Other definitions for deny that I've seen before include "Refuse to admit the truth" , "Repudiate, disavow" , "Refuse to grant" , "Refute, contradict" , "Reject as true" .)