These athletes are European - true Slav extraction (4,8)

I believe the answer is:

pole vaulters

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'these athletes' is the definition.
(pole vaulter is a kind of athlete)

'european true slav extraction' is the wordplay.
'european' becomes 'Pole' (someone from Poland).
'true slav extraction' becomes 'vaulters' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'pole'+'vaulters'='POLE-VAULTERS'

'are' is the link.

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