Diagrammatically, London's thin red line (7)
I believe the answer is:
central
'line' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for central that I've seen before include "Extremely important" , "Every answer here has one such letter" , "Key" , "like this answer" , "Most important, principal" .)