Being critical is essential (5)
I believe the answer is:
vital
This clue is a double definition.
'being critical' is the first definition.
('vital' can be a synonym of 'critical')
'essential' is the second definition.
(similar in meaning)
'is' is the link.
(Other definitions for vital that I've seen before include "Essential for life; key" , "Critical, necessary" , "Highly important" , "Of key importance" , "Urgently needed or absolutely necessary" .)