Cunningly altering figure (8)
I believe the answer is:
triangle
'figure' is the definition.
(I know that triangle is a type of polygon)
'cunningly altering' is the wordplay.
'cunningly' indicates an anagram (a cunning or tricky form of the letters).
'altering' anagrammed gives 'TRIANGLE'.
(Other definitions for triangle that I've seen before include "Polygon, could be isosceles" , "Three-sided figure (maths)" , "diamond cut in half?" , "Geometric shape (eternal, maybe)" , "Integral (anag) - geometrical shape" .)