"Humbug!" lead-in (3)
I believe the answer is:
bah
'humbug lead-in' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for bah that I've seen before include "''... Humbug'' Scrooge's cry" , "Expression of disgust or contempt" , "Appalling!" , "Exclamation of contempt or disgust - Scrooge used to say it" , "Exclamation of disgust or annoyance" .)