Basic, fundamental (6)
I believe the answer is:
primal
This clue is a double definition.
'basic' is the first definition.
(I've seen this in another clue)
'fundamental' is the second definition.
(I know that fundamental can be written as primal)
(Other definitions for primal that I've seen before include "Original or primitive" , "Primeval, original" , "Relating to an early stage of evolution" , "Original - basic" , "Original, fundamental" .)