Fifth baby, wanting eight perhaps (4)
I believe the answer is:
cube
'eight perhaps' is the definition.
(eight is two cubed)
I can't explain the remainder of the clue.
(Other definitions for cube that I've seen before include "Regular 3-D shape" , "Geometrical solid" , "Three-dimensional shape with six sides" , "Regular hexahedron" , "Shape made of six equal squares" .)