Mark closely - a bent one needs straightening (4,4)

I believe the answer is:

nota bene

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

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

'closely a bent one needs straightening' is the wordplay.
'a bent one' anagrams to 'NOTA BENE'.
But, I am unsure how the anagram is indicated.

Can you help me to learn more?

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

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

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