Doctor dupes duck? Not genuine! (6)
I believe the answer is:
pseudo
'not genuine' is the definition.
(I've seen this before)
'doctor dupes duck?' is the wordplay.
'doctor' is an anagram indicator.
'duck?' becomes 'o' (resembles zero - 'duck' in cricket).
'dupes' is an anagram of 'pseud'.
'pseud'+'o'='PSEUDO'
(Other definitions for pseudo that I've seen before include "Not genuine" , "Spurious" , "Prefix meaning false; o dupes!" , "Use pod (anag.)" , "Ersatz" .)