Very proficient (5)
I believe the answer is:
adept
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">very proficient</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for adept that I've seen before include "One who is very skilful" , "One very skilled at work" , "Very skilful, proficient" , "Having the required skills" , "Master" .)