Pretend not to understand mime (3,4)

I believe the answer is:

act dumb

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'understand mime' is the definition.
I don't know anything about this answer so I cannot tell whether this works.

'pretend not' is the wordplay.
'pretend' becomes 'act' (I've seen this in other clues).
'not' becomes 'dumb' (I can't explain this - if you can you should give a lot more credence to this answer).
'act'+'dumb'='ACT DUMB'

'to' is the link.

Can you help me to learn more?

I've seen this clue in The Globe and Mail.

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