Crossword Genius

Only something short will go in its shell (3)

Ross

I believe the answer is:

nut

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'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" .)

I've seen this clue in the Evening Standard.
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