Specialist (6)
I believe the answer is:
expert
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">specialist</span>' is the definition.<br/ ><span class="explanation-format__explanation">(a specialist is an expert in a field)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for expert that I've seen before include "Clever" , "Person with special knowledge or ability" , "One practised" , "One who knows his stuff" , "Proficient and skilful" .)