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