Young Nicklaus earned this in leading positions (5,6)

I believe the answer is:

green jacket

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'earned this in leading positions' is the definition.
I know nothing about this answer so I cannot tell whether this works.

'young nicklaus' is the wordplay.
'young' becomes 'green' (green can mean young or inexperienced).
'nicklaus' becomes 'jacket' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'green'+'jacket'='GREEN JACKET'

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