Pivotal (8)
'pivotal' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for key that I've seen before include "Coral reef off southern coast of Florida" , "Vital" , "Lock-opener" , "Piano lever" , "Crucially important" .)
'pivotal' is the definition.
(thesaurus)
This is the entire clue.
(Other definitions for decisive that I've seen before include "Settling something conclusively, as battle may be" , "Showing clear judgment and firmness" , "Producing a definite result" , "Able to make up one's mind quickly and with firmness" , "Settling an issue quickly" .)
(Other definitions for seminal that I've seen before include "Of beginnings or early developments" , "Strongly influencing future developments" , "Strongly influential" , "Highy important" , "Menials (anag) - creative" .)