Lent; not lento (4)

I believe the answer is:

fast

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'not lento' is the definition.
(I've seen this before)

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

(Other definitions for fast that I've seen before include "Securely fixed" , "Rapid; abstain from food" , "Period during which fare is reduced" , "Rapid - diet" , "'Quickly, rapidly (4)'" .)

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

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