Relating form to shape (8)
I believe the answer is:
triangle
'to shape' is the definition.
(triangle is a kind of shape)
'relating form' is the wordplay.
'form' indicates anagramming the letters (another form of the letters).
'relating' with letters rearranged gives 'TRIANGLE'.
(Other definitions for triangle that I've seen before include "diamond cut in half?" , "Three-sided figure (maths)" , "Large tin (anag.)" , "Simple percussion instrument" , "involved in eternal emotional conflict" .)