Most proficient (6)
I believe the answer is:
ablest
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">most proficient</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for ablest that I've seen before include "Most proficient" , "Most suitable" , "Most competent in stable" , "Like pupils in top stream" , "Are highly favoured like" .)