Score roughly with header from youth — net's broken (9)

I believe the answer is:

twentyish

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'score roughly' is the definition.
The definition suggests an adjective which matches the answer.

'with header from youth net's broken' is the wordplay.
'header from' indicates taking the first letters.
'broken' is an anagram indicator.
The initial letter of 'youth' is 'y'.
'with'+'y'+'nets'='withynets'
'withynets' is an anagram of 'TWENTYISH'.

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I've seen this clue in The Times.

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