An extent of surface; a field of study (4)
I believe the answer is:
area
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">an extent of surface</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >'<span class="explanation-format__original">a field of study</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(Other definitions for area that I've seen before include "Field -- expanse" , "Extent of a 2-dimensional surface" , "Geographical region" , "The measure of a surface" , "Field of activity or surface" .)