Old Nick rested on one (5)
I believe the answer is:
satan
'old nick' is the definition.
(I know that Satan is a old nick)
'rested on one' is the wordplay.
'rested' becomes 'sat' (to sit is to rest).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'one' becomes 'an' (an apple is one apple).
'sat'+'an'='SATAN'
(Other definitions for satan that I've seen before include "Prince of Darkness" , "Apollyon" , "Fiend" , "Old Nick, Lucifer or Beelzebub" , "Diabolical figure" .)