Whole triangle is redesigned (8)
I believe the answer is:
integral
'whole' is the definition.
('integral' can be similar in meaning to 'whole')
'triangle is redesigned' is the wordplay.
'is redesigned' indicates an anagram.
'triangle' with letters rearranged gives 'INTEGRAL'.
(Other definitions for integral that I've seen before include "Fundamental; function" , "Altering so as to be necessary to the whole" , "Necessary as part of a whole, like triangle" , "United" , "Forming part of the whole" .)