But, when extra, it’s surprising (8)
I believe the answer is:
ordinary
This might be a double definition.
'but when extra' is the first definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe there's an association between them I don't understand?
'surprising' is the second definition.
Both the definition and answer are adjectives. Maybe you can see a link between them that I don't see?
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for ordinary that I've seen before include "Normal, mundane" , "old bicycle" , "simple charge" , "Normal or average" , "'Usual, everyday (8)'" .)