Plant's day off (4)
I believe the answer is:
dill
'plant's' is the definition.
(dill is a kind of plant)
'day off' is the wordplay.
'day' becomes 'd'.
'off' becomes 'ill' (**).
'd'+'ill'='DILL'
(Other definitions for dill that I've seen before include "Plant of the celery family; leaf is a herb, seeds a spice" , "Herb used in cooking" , "Herb for gravadlax" , "Herb of parsley family" , "Fool" .)