Plant with clover-like leaves (7)
I believe the answer is:
'plant with clover-like leaves' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for trefoil that I've seen before include "A flower; three-lobe ornament" , "Three-lobed leaf" , "Small clover-like plant" , "Three leaved plant like clover and shamrock" , "clover, possibly" .)