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