Complete reinvention of triangle (8)
I believe the answer is:
integral
'complete' is the definition.
(I've seen this in another clue)
'reinvention of triangle' is the wordplay.
'reinvention of' is an anagram indicator (I've seen this in other clues).
'triangle' with letters rearranged gives 'INTEGRAL'.
(Other definitions for integral that I've seen before include "Relating to being complete" , "Belonging as an essential part" , "Built-in -- 1 across (anag)" , "United" , "Forming an essential part" .)