Dupes are baffled by fake (5)
I believe the answer is:
pseud
'fake' is the definition.
(I know that fake can be written as pseud)
'dupes are baffled' is the wordplay.
'are baffled' is an anagram indicator.
'dupes' is an anagram of 'PSEUD'.
'by' acts as a link.
(Other definitions for pseud that I've seen before include "Socially pretentious person" , "Pretentious type" , "Pretentious fraud" , "Pretentious person (colloq.)" , "Fake intellectual" .)