Smarmy thief concealing lie (4)
I believe the answer is:
myth
'lie' is the definition.
(thesaurus)
'smarmy thief concealing' is the wordplay.
'concealing' says the answer is hidden in the clue.
'MYTH' can be found hidden inside 'smarmy thief'.
(Other definitions for myth that I've seen before include "Legend, folk tale" , "don't believe a word of it!" , "Common false belief" , "(Urban?) story" , "Wildly-held but false belief or idea" .)