Old Nick sat on one (5)
I believe the answer is:
satan
'old nick' is the definition.
('Satan' can be a synonym of 'old nick')
'sat on one' is the wordplay.
'on' means one lot of letters go next to another.
'one' becomes 'an' (I've seen this in other clues).
'sat'+'an'='SATAN'
(Other definitions for satan that I've seen before include "Prince of Darkness" , "arch-fiend" , "Fallen archangel" , "Diabolical figure" , "Old Nick, Lucifer or Beelzebub" .)