Basic shape: triangle (8)
I believe the answer is:
integral
'basic' is the definition.
(I've seen this in another clue)
'shape triangle' is the wordplay.
'shape' indicates anagramming the letters.
'triangle' anagrammed gives 'INTEGRAL'.
(Other definitions for integral that I've seen before include "United" , "Triangle/relating/altering/alerting (anag)" , "contributing to unity" , "Fundamental; function" , "though whole" .)