Spell; rhythm (4)
I believe the answer is:
time
This clue is a double definition.
'spell' is the first definition.
(spell can mean a period of time)
'rhythm' is the second definition.
(time is a kind of rhythm)
(Other definitions for time that I've seen before include "Prison sentence; tempo" , ""The enemy"" , "Speech to close pub" , "Hours clocked up?" , "Rhythm" .)