Endlessly racing round a plant (6)
I believe the answer is:
arnica
'plant' is the definition.
(arnica is a kind of plant)
'endlessly racing round a' is the wordplay.
'endlessly' means to remove the last letter.
'round' indicates an anagram.
'racing' with its final letter taken away is 'racin'.
'racin' anagrammed gives 'arnic'.
'arnic'+'a'='ARNICA'
(Other definitions for arnica that I've seen before include "Ointment used to treat bruises" , "Plant used to treat bruises" , "Tincture of mountain tobacco, good for bruises, etc" , "Tincture for treating bruises" , "Herbal ointment used on bruises" .)