Correct wordplay is hard (6)
I believe the answer is:
punish
'correct' is the definition.
(thesaurus)
'wordplay is hard' is the wordplay.
'wordplay' becomes 'pun' (synonyms).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'pun'+'is'+'h'='PUNISH'
(Other definitions for punish that I've seen before include "Fine perhaps" , "Discipline" , "Inflict a penalty upon" , "exact retribution" , "Flog" .)