Fluid altering shape (8)
I believe the answer is:
triangle
'shape' is the definition.
(triangle is a kind of shape)
'fluid altering' is the wordplay.
'fluid' indicates an anagram.
'altering' anagrammed gives 'TRIANGLE'.
(Other definitions for triangle that I've seen before include "Integral (anag) - figure" , "Alerting (anag)" , "Three-sided shape" , "Constellation - three-sided figure" , "Geometrical figure - musical instrument" .)