Nibble cheek once taken aback (4)

I believe the answer is:

gnaw

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'nibble' is the definition.
(I've seen this in another clue)

I don't understand how the rest of the clue works.

(Other definitions for gnaw that I've seen before include "Chew at, work with teeth" , "Bite like a rodent" , "Nibble like a rodent" , "What rodents typically do" , "Chew steadily at" .)

I've seen this clue in The Times.

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