Trefoil (6)
I believe the answer is:
clover
'trefoil' is the definition.
(synonyms)
This is all the clue.
(Other definitions for clover that I've seen before include "A four-leafed one is lucky" , "Weed or forage plant (which it's good to be in)" , "Meadow plant" , "A 19 down" , "Another trefoil, sometimes with lucky four leaves" .)