Whistle-blower interrupting work in plant (7)
I believe the answer is:
trefoil
'plant' is the definition.
(trefoil is a kind of plant)
'whistle-blower interrupting work' is the wordplay.
'whistle blower' becomes 'ref' (I've seen this in other clues).
'interrupting' indicates putting letters inside.
'work' becomes 'toil' (toiling is a kind of working**).
'ref' placed into 'toil' is 'TREFOIL'.
'in' acts as a link.
(Other definitions for trefoil that I've seen before include "Loftier (anag) -- clover-like plant" , "Three-leaf clover" , "Three-lobed leaf" , "Clover leaf" , "clover, possibly" .)