Critical (3)

I think it could be either:

Which length description are you looking for?:

key

'critical' is the definition.
(key can mean very important)

This is the entire clue.

(Other definitions for key that I've seen before include "Clue" , "Of crucial importance" , "Lock undoer" , "Typewriter button, isle" , "//www.crosswordgiant.com/crossword-clue/1252911/" .)

I've seen this clue in The Telegraph and The Wall Street Journal.

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