At the heart of the tube system (7)
I believe the answer is:
central
This might be a double definition.
'at' is the first definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'heart of the tube system' is the second definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'the' acts as a link.
Can you help me to learn more?
(Other definitions for central that I've seen before include "Focal" , "Key" , "Middle; crucial" , "Every answer here has one such letter" , "like this answer" .)