Form solid pile in solid form (9)
I believe the answer is:
ellipsoid
'solid form' is the definition.
'form solid pile' is the wordplay.
'form' indicates anagramming the letters.
'solid'+'pile'='solidpile'
'solidpile' with letters rearranged gives 'ELLIPSOID'.
'in' is the link.
(Other definitions for ellipsoid that I've seen before include "The earth is of this shape" , "solid figure?" .)