Least exciting
'least exciting' is the definition.
(I've seen this before)
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" .)