I believe the answer is:
'hardback' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps 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 stern that I've seen before include "Grimly serious; back" , "Severe, forbidding" , "Unsmiling" , "Looking severe" , "Harsh at the back of the ship" .)