Least expensive (8)
I believe the answer is:
cheapest
'least expensive' is the definition.
('cheap' is the opposite of 'expensive' so 'cheapest' should be a synonym for 'least expensive')
This is the entire clue.
(Other definitions for cheapest that I've seen before include "The space (anag.)" , "Of the poorest quality" , "Most economical" , "Most cut-price" , "Most inferior" .)