Possibly altering a figure (8)
I believe the answer is:
triangle
'a figure' is the definition.
(I know that triangle is a type of polygon)
'possibly altering' is the wordplay.
'possibly' is an anagram indicator.
'altering' is an anagram of 'TRIANGLE'.
(Other definitions for triangle that I've seen before include "Integral part of common geometric figure" , "Polygon, could be isosceles" , "Percussion instrument of geometric origin" , "Three-sided figure - musical instrument" , "It has three sides" .)