Essential to have triangle repositioned (8)
I believe the answer is:
integral
'essential' is the definition.
(I've seen this before)
'triangle repositioned' is the wordplay.
'repositioned' indicates an anagram.
'triangle' anagrammed gives 'INTEGRAL'.
'to have' acts as a link.
(Other definitions for integral that I've seen before include "Maths function; whole" , "Built-in -- 1 across (anag)" , "using no fractions" , "Basic" , "United" .)