It’s good when twice the usual length (4)

I believe the answer is:

okay

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'it's good' is the definition.
(thesaurus)

I can't explain the rest of the clue.

(Other definitions for okay that I've seen before include "Give approval to" , "Correct" , "All right; sanction" , "Approve, endorse" , "Permissible (colloq.)" .)

I've seen this clue in the Evening Standard.

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