Basic triangle redrawn (8)
I believe the answer is:
integral
'basic' is the definition.
(thesaurus)
'triangle redrawn' is the wordplay.
'redrawn' indicates anagramming the letters (I've seen this in other clues).
'triangle' anagrammed gives 'INTEGRAL'.
(Other definitions for integral that I've seen before include "whole thing" , "A component and indispensable part of, like triangle" , "Maths function; entire" , "Forming part of the whole" , "Of part necessary to the completeness of a whole" .)