Super important (3)
(Other definitions for key that I've seen before include "Essential; solution" , "Eg, Ctrl, Alt or Esc" , "//www.crosswordgiant.com/crossword-clue/1252911/" , "It opens the door" , "Typewriter button, isle" .)
I believe this clue is a double definition.
'super' is the first definition.
(I've seen this in another clue)
'important' is the second definition.
(similar in meaning)
(Other definitions for great that I've seen before include "Extensive; excellent" , "Excellent (colloq.)" , "Very good, excellent" , "The Plough" , "Of considerable size" .)