Heartbeat (5)
I believe the answer is:
pulse
'heartbeat' is the definition.
(I know that heartbeat can be written as pulse)
This is the entire clue.
(Other definitions for pulse that I've seen before include "Heart-throb" , "Seed of bean or pea plant" , "Lentil, bean, seeds" , "Regular rhythm" , "often sought medically?" .)