Beat rhythmically (5)
I believe the answer is:
throb
'beat rhythmically' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for throb that I've seen before include "Drum" , "Strong rhythmic beat" , "Vibrate with rhythm like heart" , "Martin -; Kingsley -" , "Pulsate with force" .)