Basic triangle formation (8)
I believe the answer is:
integral
'basic' is the definition.
(I've seen this in another clue)
'triangle formation' is the wordplay.
'formation' is an anagram indicator.
'triangle' anagrammed gives 'INTEGRAL'.
(Other definitions for integral that I've seen before include "Necessary as part of a whole, like triangle" , "Triangle being essential" , "form whole" , "using no fractions" , "Forming part of the whole" .)