Strange altering shape (8)
I believe the answer is:
triangle
'shape' is the definition.
(I know that triangle is a type of shape)
'strange altering' is the wordplay.
'strange' indicates an anagram.
'altering' with letters rearranged gives 'TRIANGLE'.
(Other definitions for triangle that I've seen before include "Geometric shape (eternal, maybe)" , "complex romantic situation?" , "Regular shape with" , "Polygon, could be isosceles" , "Three-sided figure (maths)" .)