Kop supporter? (8)

I believe the answer is:

keystone

I believe this is a double definition.

'kop' is the first definition.
(I've seen this before)

'supporter?' is the second definition.
(I've seen this before)

(Other definitions for keystone that I've seen before include "Fundamental item in a structure" , "Central part of a system" , "everything may depend on it" , "Locking piece for arch" , "Central block at the top of an arch" .)

I've seen this clue in The Sun.

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