Basic — essential (7)
I believe the answer is:
primary
I believe this clue is a double definition.
'basic' is the first definition.
(thesaurus)
'essential' is the second definition.
(associated in meaning)
(Other definitions for primary that I've seen before include "The colours from which all others can be made" , "Paramount" , "Cardinal" , "Principal" , "Of the first order" .)