Arch-supporter of comic policemen (8)
I believe the answer is:
keystone
'arch' is the definition.
(thesaurus)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for keystone that I've seen before include "Central principle of a policy" , "Something on which all else depends" , "Central part of a system" , "Piece at the top of a masonry arch" , "Kop" .)