At heart (7)
I believe the answer is:
central
'at heart' 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?
This is all the clue.
Can you help me to learn more?
(Other definitions for central that I've seen before include "Focal" , "Most important, principal" , "Near the middle" , "Middle; crucial" , "Every answer here has one such letter" .)