Crossword Genius

Sound indication that a watch is fully wound up? (5,5)

Ross

I believe the answer is:

eight bells

I'm a little stuck... Click here to teach me more about this clue!

'indication that a watch is fully wound up?' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'sound' is the wordplay.
I cannot quite see how this works, but
'sound' could be 'bell' (bell is a kind of sound) and 'bell' is present in the answer.
The remaining letters 'eights' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in The Guardian and The Globe and Mail.
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