Triangle turned out to be fundamental (8)
I believe the answer is:
integral
'to be fundamental' is the definition.
(I've seen this before)
'triangle turned' is the wordplay.
'turned' is an anagram indicator.
'triangle' anagrammed gives 'INTEGRAL'.
'out' acts as a link.
(Other definitions for integral that I've seen before include "Maths function; whole" , "Belonging as an essential part" , "form whole" , "Forming part of the whole" , "Triangle (anag)" .)