Like a play on words lacking core discipline (6)
I believe the answer is:
punish
'discipline' is the definition.
(I've seen this before)
'like a play on words lacking core' is the wordplay.
'like a play on words' becomes 'punnish' (like a pun).
'lacking core' says to hollow out the word (remove centre letters).
'punnish' with its centre removed is 'PUNISH'.
(Other definitions for punish that I've seen before include "Like wordplay in speech" , "Penalise" , "Possibly fine" , "Fine maybe" , "Sentence" .)