Most ancient (6)
I believe the answer is:
oldest
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">most ancient</span>' is the definition.<br/ ><span class="explanation-format__explanation">('old' can be similar in meaning to 'ancient')</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for oldest that I've seen before include "Most aged" , "Eldest" , "Most senior in age" , "Least young" , "Most ancient (O, not E)" .)