Oven base erected - plus lining (7)
I believe the answer is:
tandoor
'oven' is the definition.
(type of Asian oven)
'base erected plus lining' is the wordplay.
'base' becomes 'root' (synonyms).
'erected' says the letters should be written backwards (in a down clue, letters go up).
'plus' becomes 'and' (I've seen this in another clue).
'lining' means one lot of letters goes inside another.
'root' backwards is 'toor'.
'toor' enclosing 'and' is 'TANDOOR'.
(Other definitions for tandoor that I've seen before include "It has food cooked" , "Indian oven" , "Clay oven" .)