Allow to escape (3,4)

I think it could be either:

Which length description are you looking for?:

3,4

let slip

(Other definitions for let slip that I've seen before include "Fail to keep Secret" , "Reveal accidentally, or allow to skid?" , "Allow to escape (information etc)" , "Allow to fall or reveal a secret accidentally" , "accidentally revealed" .)

I've seen this clue in the Evening Standard.

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