Girl stretches sister just a little bit (7)

I believe the answer is:

nucleon

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'a little bit' is the definition.
The answer and definition can be both natural objects as well as being singular nouns.
Perhaps they are linked in a way I don't understand?

'girl stretches sister' is the wordplay.
I cannot really understand how this works, but
'girl' could be 'cleo' and 'cleo' is found in the answer.
'sister' could be 'nun' (sister is a term of address for a nun) and 'nun' is found within the remaining letters.
No letters remain.
This explanation may well be incorrect...

'just' acts as a link.

Can you help me to learn more?

(Other definitions for nucleon that I've seen before include "fundamental constituent" , "Particle" .)

I've seen this clue in the Sydney Morning Herald.

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