It has a keystone (4)
I believe the answer is:
'it has a keystone' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for arch that I've seen before include "Supporting structure" , "Affectedly playful like senior angel" , "Mischievous - shrewd" , "Mischievous - curve" , "Devilish" .)