Least difficult (7)
I believe the answer is:
easiest
'least difficult' is the definition.
('easy' is the opposite of 'difficult' so 'easiest' should be a synonym for 'least difficult')
This is all the clue.
(Other definitions for easiest that I've seen before include "Posing least difficulty" , "most relaxed" , "Most convenient" , "Requiring the least amount of effort" , "most amenable" .)