Insincerity not working in part of Switzerland (4)
I believe the answer is:
cant
'insincerity' is the definition.
(I've seen this before)
'not working in part of switzerland' is the wordplay.
'not' indicates named letters should be taken away.
'working in' becomes 'on' (I've seen this in other clues).
'part of switzerland' becomes 'canton' (Swiss administrative subdivision).
'canton' with 'on' taken away is 'CANT'.
(Other definitions for cant that I've seen before include "Affected talk" , "tilt" , "Type 16 [OF SPEECH]" , "Insincere talk, pious platitudes" , "Pious platitudes -- jargon" .)