Cannot reduce insincerity (4)
I believe the answer is:
cant
'insincerity' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for cant that I've seen before include "Not able to" , "Incline, slope" , "Affected use of moral sentiments" , "Type 16 [OF SPEECH]" , "Hypocritical talk sounds unable" .)