Crossword Genius

Allow, but with constraint (3)

Ross

I believe the answer is:

let

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

This clue might be a double definition.

'allow' is the first definition.
(I know that allow is a more specific form of allow)

'with constraint' is the second definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

'but' is the link.

Can you help me to learn more?

(Other definitions for let that I've seen before include "Permitted" , "hired" , "Allow on the tennis court" , "Service that must be repeated" , "Allowed or rented out" .)

I've seen this clue in the Evening Standard.
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