One doing less well than expected (13)
I believe the answer is:
underachiever
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">one doing less well than expected</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 underachiever that I've seen before include "One doing less well than expected" , "Person not reaching their potential" .)