Regular beat (5)
'regular beat' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for rhythm that I've seen before include "Musical pulse" , "Musical tempo or beat" , "Sense of a steady beat" , "Regular beat in music" , "Measured beat" .)
(Other definitions for pulse that I've seen before include "Green" , "flageolet?" , "Heart rhythm" , "Rhythmical throb of arteries" , "Rhythmic throbbing" .)