Spanish champion, the Cid, beheaded by pellet (8)
I believe the answer is:
spheroid
'pellet' is the definition.
(thesaurus)
'spanish champion the cid beheaded' is the wordplay.
'spanish' becomes 'sp'.
'champion' becomes 'hero' (synonyms).
'beheaded' means to remove the first letter (the head of the word is removed).
'cid' with its initial letter removed is 'id'.
'sp'+'hero'+'id'='SPHEROID'
'by' is the link.
(Other definitions for spheroid that I've seen before include "Almost the shape of a ball" , "Kind of globe, not perfectly round" , "Solid" , "Almost-perfect-ball shape" , "Round shape" .)