least gradual (8)
I believe the answer is:
steepest
'least gradual' is the definition.
('steep' is the opposite of 'gradual' so 'steepest' should be a synonym for 'least gradual')
This is the entire clue.
(Other definitions for steepest that I've seen before include "Rising most sharply" , "extremely dear" , "Most expensive" , "most severe" , "With the sharpest incline" .)