Old Harry was enthroned on one (5)
I believe the answer is:
satan
'old harry' is the definition.
(I've seen this in another clue)
'enthroned on one' is the wordplay.
'enthroned on' becomes 'sat' (I can't explain this - if you can you should believe this answer much more).
'one' becomes 'an' (an apple is one apple).
'sat'+'an'='SATAN'
'was' acts as a link.
Can you help me to learn more?
(Other definitions for satan that I've seen before include "arch-fiend" , "Prince of Darkness" , "Old Nick, Lucifer or Beelzebub" , "Fiend" , "The devil, Beelzebub" .)