It has a keystone (4)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">it has a keystone</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 arch that I've seen before include "Supporting structure" , "Affectedly playful like senior angel" , "Mischievous - shrewd" , "Mischievous - curve" , "Devilish" .)