At the heart of London's thin red line! (7)
I believe the answer is:
central
'line' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe they are linked in a way I don't understand?
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" , "Focal" , "Near the middle" , "Every answer here has one such letter" , "like this answer" .)