Play on words is hard to beat (6)
I believe the answer is:
punish
'beat' is the definition.
(I've seen this before)
'play on words is hard' is the wordplay.
'play on words' becomes 'pun' (synonyms).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'pun'+'is'+'h'='PUNISH'
'to' is the link.
(Other definitions for punish that I've seen before include "beat" , "Sentence" , "exact retribution" , "Castigate" , "Impose penalty for crime or misbehaviour" .)