Hard work put an end to child's play (9)
I believe the answer is:
hopscotch
'child's play' is the definition.
(I've seen this before)
'hard work put an end to' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'work' becomes 'op' (abbreviation for opus).
'put an end to' becomes 'scotch' (I've seen this before).
'h'+'op'+'scotch'='HOPSCOTCH'
(Other definitions for hopscotch that I've seen before include "Children's pavement game" , "Playground game" , "Children's game with squares on ground" , "child's play" , "minor diversion?" .)