Semicircular recess (4)
I believe the answer is:
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<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">semicircular recess</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 apse that I've seen before include "room for an altar" , "Recess in church wall" , "Domed section of a church" , "space for altar" , "Semicircular end of a church, behind the altar" .)