Castigate young creature breaking seat (8)
I believe the answer is:
lambaste
'castigate' is the definition.
(thesaurus)
'young creature breaking seat' is the wordplay.
'young creature' becomes 'lamb' (young sheep).
'breaking' indicates anagramming the letters.
'seat' is an anagram of 'aste'.
'lamb'+'aste'='LAMBASTE'
(Other definitions for lambaste that I've seen before include "Berate severely" , "carpet" , "Severely criticise" , "Beat -- reprimand severely" , "Censure severely or angrily" .)