Only something short will go in its shell (3)
I believe the answer is:
nut
'shell' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe they are linked in a way I don't understand?
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for nut that I've seen before include "Hard fruit or informal lunatic" , "Hard shell of a locking device" , "Fanatic (slang)" , "Enthusiastic person" , "eccentric type" .)