Throbbing; plant (5)
I believe the answer is:
pulse
I believe this clue is a double definition.
'throbbing' is the first definition.
(I have seen 'Rhythmic throbbing ' mean 'pulse' so perhaps 'throbbing' could also mean 'pulse')
'plant' is the second definition.
(as in a plant which produces pulses)
(Other definitions for pulse that I've seen before include "runner, possibly" , "Arterial throb" , "Vitality" , "Throb, beans" , "Green" .)