Cannot reduce insincerity (4)

I believe the answer is:

cant

I'm a little stuck... Click here to teach me more about this clue!

This clue could be a double definition.

'cannot' is the first definition.
(contraction)

'insincerity' is the second definition.
(I've seen this before)

'reduce' acts as a link.
I am not very happy about this link. It may belong to one of the definitions.

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" .)

I've seen this clue in The Mirror.

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