Cops at top of arch (8)
I believe the answer is:
keystone
'arch' is the definition.
(thesaurus)
'cops at top' is the wordplay.
I cannot quite understand how this works, but
'at' could be 'on' (location) and 'on' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
'of' acts as a link.
Can you help me to learn more?
(Other definitions for keystone that I've seen before include "Something on which all else depends" , "Central principle of a policy" , "Vital support" , "Central policy; arch top" , "Locking piece for arch" .)