Field of operation (6)
I believe the answer is:
sphere
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">field of operation</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for sphere that I've seen before include "Solid figure - range of activity" , "Round shape" , "Round solid figure with every point on its surface equidistant from its centre" , "Ball, orb" , "All-rounder" .)