Decisive, critical (7)
I believe the answer is:
crucial
I believe this is a double definition.
'decisive' is the first definition.
(associated in meaning)
'critical' is the second definition.
(associated in meaning)
(Other definitions for crucial that I've seen before include "Pivotal" , "All-important" , "Of utmost importance" , "Of extreme importance" , "'Decisive, critical (7)'" .)