Fast American runner or weightlifter, poor sportsman (10)

I believe the answer is:

jackrabbit

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

'fast american runner or weightlifter' is the definition.
I can't tell whether this defines the answer.

'poor sportsman' is the wordplay.
'poor' becomes 'jack' (I can't justify this - if you can you should believe this answer much more).
'sportsman' becomes 'rabbit' (I have seen 'Inferior sportsman' mean 'rabbit' so perhaps 'sportsman' could also mean 'rabbit').
'jack'+'rabbit'='JACKRABBIT'

Can you help me to learn more?

I've seen this clue in The Guardian.

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