Film cops' internal complaint when current defamation case is thrown out (8)
I believe the answer is:
keystone
'film' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe they are linked in a way I don't understand?
I don't understand how the remainder of the clue works.
(Other definitions for keystone that I've seen before include "Piece at the top of a masonry arch" , "Central part of arch" , "Vital support" , "supporter?" , "everything may depend on it" .)