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

'nicklaus earned this in leading positions' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.

'young' is the wordplay.
I cannot really understand how this works, but
'young' could be 'green' (green can mean young or inexperienced) and 'green' is found within the answer.
The remaining letters 'jacket' 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?