Very small pieces swapped in USS Nautilus? (10)

'very small pieces swapped' is the definition.
I know nothing about this answer so I cannot judge whether this works.

'uss nautilus?' is the wordplay.
I cannot quite see how this works, but
'nautilus?' could be 'sub' (nautilus is a kind of sub) and 'sub' is present in the answer.
The remaining letters 'nuclear' is a valid word which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).

'in' is the link.

Can you help me to learn more?