Vaulted alcove (4)
I believe the answer is:
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<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">vaulted alcove</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 apse that I've seen before include "place in church" , "part of cathedral" , "Rounded end of building" , "Strange peas in the church recess" , "Recess with domed rof in a church" .)