Pupil once turning eleven, no good for sport (6)
I believe the answer is:
boxing
'sport' is the definition.
(boxing is a kind of sport)
'pupil once turning eleven no good' is the wordplay.
'pupil once' becomes 'ob'.
'turning' shows that the letters should be reversed in order.
'eleven' becomes 'xi' (roman numeral).
'no' becomes 'n' (common abbreviation).
'good' becomes 'g' (abbreviation).
'ob' reversed gives 'bo'.
'bo'+'xi'+'n'+'g'='BOXING'
'for' is the link.
(Other definitions for boxing that I've seen before include "Sport of Bruno, Tyson and Lewis" , "Packer's job" , "Combat sport" , "The noble art?" , "Fighting in a ring" .)