Plant different encased in lead (3,4)
I believe the answer is:
pot herb
'plant' is the definition.
(type of plant)
'different encased in lead' is the wordplay.
'different' becomes 'other' (similar in meaning).
'encased in' means one lot of letters goes inside another.
'lead' becomes 'Pb' (Pb is the chemical symbol for lead).
'other' placed within 'pb' is 'POT-HERB'.
(Other definitions for pot herb that I've seen before include "culinary plant" , "sage, perhaps" , "Vegetable adding flavour" .)