Old Harry was seated on one (5)
I believe the answer is:
satan
'old harry' is the definition.
(I've seen this in another clue)
'seated on one' is the wordplay.
'seated' becomes 'sat' ('sit' can be a synonym of 'seat').
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'one' becomes 'an' (an apple is one apple).
'sat'+'an'='SATAN'
'was' is the link.
(Other definitions for satan that I've seen before include "Old Nick, Lucifer or Beelzebub" , "Diabolical figure" , "Prince of Darkness" , "The devil, Beelzebub" , "Apollyon" .)