Most faithful
I believe the answer is:
truest
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">most faithful</span>' is the definition.<br/ ><span class="explanation-format__explanation">('true' can be similar in meaning to 'faithful')</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for truest that I've seen before include "Most loyal or veracious" , "Most honest" , "Utters in most straight way" , "Most accurate" , "Most reliable" .)