Least wild (6)
I believe the answer is:
tamest
'least wild' is the definition.
('tame' is the opposite of 'wild' so 'tamest' should be a synonym for 'least wild')
This is the entire clue.
(Other definitions for tamest that I've seen before include "Least wild, most domesticated" , "Most docile" , "Meekest or least wild" , "Most restrained or quiet" , "Least exciting" .)