Badly spoiled one line in solid figure (9)
I believe the answer is:
ellipsoid
'solid figure' is the definition.
(geometric figure)
'badly spoiled one line' is the wordplay.
'badly' indicates an anagram (letters in the wrong order).
'one' becomes 'i' (Roman numeral).
'line' becomes 'l' (used when specifying particular lines from a poem).
'spoiled'+'i'+'l'='spoiledil'
'spoiledil' 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?" .)