Plant where whistler comes into work (7)
I believe the answer is:
trefoil
'plant' is the definition.
(I know that trefoil is a type of plant)
'whistler comes into work' is the wordplay.
'whistler' becomes 'ref' (a referee blows a whistle).
'comes into' is an insertion indicator.
'work' becomes 'toil' (toil is a kind of work).
'ref' placed into 'toil' is 'TREFOIL'.
'where' acts as a link.
(Other definitions for trefoil that I've seen before include "Clover leaf" , "A flower; three-lobe ornament" , "clover, possibly" , "Small clover-like plant" , "Leaf divided into three sections" .)