Dupes criminal with old sham (6)
I believe the answer is:
pseudo
'sham' is the definition.
(I know that sham can be written as pseudo)
'dupes criminal with old' is the wordplay.
'criminal with' is an anagram indicator.
'old' becomes 'o' (abbreviation).
'dupes' anagrammed with 'o' makes 'PSEUDO'.
(Other definitions for pseudo that I've seen before include "O dupes! being false and insincere" , "Prefix meaning false; o dupes!" , "Fake, mock, false" , "artificial" , "Counterfeit" .)