Chief leader writer, fiery beast (9)
I believe the answer is:
pendragon
'chief leader' is the definition.
(name for King Uther, father of King Arthur)
'writer fiery beast' is the wordplay.
'writer' becomes 'pen' (something which writes).
'fiery beast' becomes 'dragon' (beast that breathes fire).
'pen'+'dragon'='PENDRAGON'
(Other definitions for pendragon that I've seen before include "Ruler" , "Tribal chief," , "Uther --" , "King Arthur, for example" , "old British chief" .)