Critical (3)
I believe the answer is:
key
'critical' is the definition.
(key can mean very important)
This is all the 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/" .)