London's thin red line (under ground) (7)
I believe the answer is:
central
'line under ground' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe there's a link between them I don't understand?
'london's thin red' is the wordplay.
I cannot really understand how this works, but
'red' could be 'r' (I've seen this in other clues) and 'r' is present in the answer.
The remaining letters 'cental' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for central that I've seen before include "Middle; crucial" , "Key" , "Extremely important" , "like this answer" , "Every answer here has one such letter" .)