Modifying a sort of triangle (8)
I believe the answer is:
altering
'modifying' is the definition.
(thesaurus)
'a sort of triangle' is the wordplay.
'a sort of' indicates anagramming the letters.
'triangle' anagrammed gives 'ALTERING'.
(Other definitions for altering that I've seen before include "Changing -- 1 across (anag)" , "Modifying, changing" , "making a change" , "Making changes" , "revising" .)