Milfoil (6)
I believe the answer is:
yarrow
'milfoil' is the definition.
(synonyms)
This is all the clue.
(Other definitions for yarrow that I've seen before include "Flowering plant" , "Creeping perennial plant, used in herbal medicine" , "'Achillea millefolium'" , "bloomer" , "Species of milfoil" .)