One adept at serving (4)
I believe the answer is:
acer
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">one adept at serving</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for acer that I've seen before include "Type of maple" , "Tree with winged fruit" , "Care about the maple?" , "Tree or shrub of the maple genus" , "ornamental tree" .)