Critical - full of life (5)
I believe the answer is:
vital
I believe this is a double definition.
'critical' is the first definition.
(I know that critical can be written as vital)
'full of life' is the second definition.
(synonyms)
(Other definitions for vital that I've seen before include "Energetic - displaying life" , "Urgently needed or absolutely necessary" , "Highly important" , "Essential for life; key" , "Necessary for life" .)