A trefoil plant (6)
I believe the answer is:
clover
'a trefoil plant' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for clover that I've seen before include "A four-leaved one is considered lucky" , "A four - leafed one is for luck (6)" , "Pasture plant" , "A four-leafed one is lucky" , "Green trefoil plant" .)