Least dangerous (6)
I believe the answer is:
safest
'least dangerous' is the definition.
('safe' is the opposite of 'dangerous' so 'safest' should be a synonym for 'least dangerous')
This is the entire clue.
(Other definitions for safest that I've seen before include "Most dependable" , "Most protected from danger" , "Most secure and free from danger" , "Least dangerous" , "Least risky" .)