Plant is different when covered in lead (3,4)
I believe the answer is:
pot herb
'plant' is the definition.
(type of plant)
'different when covered in lead' is the wordplay.
'different' becomes 'other' (similar in meaning).
'when covered in' is an insertion indicator.
'lead' becomes 'Pb' (Pb is the chemical symbol for lead).
'other' inserted within 'pb' is 'POT-HERB'.
'is' is the link.
(Other definitions for pot herb that I've seen before include "Vegetable adding flavour" , "sage, perhaps" , "culinary plant" .)