Least exciting (6)
'least exciting' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tamest that I've seen before include "Least wild, most domesticated" , "Most docile" , "Most restrained or quiet" , "Least exciting" , "Meekest or least wild" .)
(Other definitions for dullest that I've seen before include "that's not at all interesting" , "Least interesting" , "Least shiny" , "Most unintelligent" , "Most insensitive" .)