Impostor? Nonsense (6)
I believe the answer is:
humbug
I believe this clue is a double definition.
'impostor?' is the first definition.
(thesaurus)
'nonsense' is the second definition.
(humbug is a kind of nonsense)
(Other definitions for humbug that I've seen before include "The sweet could be pretentious nonsense" , "Peppermint confection" , "talk that's deceptive" , "Hypocrisy - bah!" , "that a sucker may enjoy" .)