Closely observe what's cut in middle of Corn Belt (4,4)

I believe the answer is:

nota bene

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'belt' is the definition.
Both the definition and answer are singular nouns.
Perhaps they are linked in a way I don't understand?

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

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(Other definitions for nota bene that I've seen before include "Pay particular attention to" , "Expansion of Latin notation N.B." , "Observe carefully (Latin)" , "'Mark well, in Latin (4,4)'" , "Look carefully" .)

I've seen this clue in The Guardian.

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