Basic; lively (5)
I believe the answer is:
vital
I believe this clue is a double definition.
'basic' is the first definition.
(thesaurus)
'lively' is the second definition.
(similar in meaning)
(Other definitions for vital that I've seen before include "Essential; lively" , "Essential to life or critically important" , "Most important, like certain body organs" , "Are such statistics" , "Of the greatest importance" .)