Vital sign (5)
I believe the answer is:
pulse
'vital sign' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for pulse that I've seen before include "Vibration" , "Throb, quiver" , "When you're gone, you haven't one" , "Excitement" , "Edible seeds - which nurse may take" .)