Lengths first spanned by balls (9)

I believe the answer is:

distances

'lengths' is the definition.
(distance is a kind of length)

'first spanned by balls' is the wordplay.
'first' becomes 'ist' (ist resembles 1st).
'spanned by' means one lot of letters goes inside another.
'balls' becomes 'dances' (I've seen this before).
'ist' put into 'dances' is 'DISTANCES'.

I've seen this clue in The Guardian.

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