"Guinness Book” suffix (3)

I believe the answer is:

est

'guinness book suffix' is the definition.
(I've seen this in another clue)

This is all the clue.

(Other definitions for est that I've seen before include "Suffix showing superlative" , "self-improvement programme" , "Time Americans adopted" , "psychological programme" .)

I've seen this clue in The New York Times and the King Feature Syndicate.

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