Cogent (5)

I think it could be either:

Which length description are you looking for?:

valid

'cogent' is the definition.
(I've seen this in another clue)

This is all the clue.

(Other definitions for valid that I've seen before include "Legally acceptable, unexpired" , "Legally in force" , "Sound and to the point, also legal" , "Effectual" , "Still legally acceptable" .)

I've seen this clue in the Evening Standard.

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