Some athlete, a seriously tough one (6)
I believe the answer is:
teaser
'tough one' is the definition.
(I've seen this in another clue)
'some athlete a seriously' is the wordplay.
'some' indicates the answer is hidden within the clue (some parts of the word are used).
'TEASER' is hidden in the letters of 'athlete a seriously'.
(Other definitions for teaser that I've seen before include "Easter (anag) - puzzle" , "Tricky question; intriguing preliminary advert" , "Riddle" , "Hard question" , "Difficult problem concerning Easter" .)