Nice one: shocker verboten (5)
I believe the answer is:
taboo
'verboten' is the definition.
(I've seen this in another clue)
'nice one shocker' is the wordplay.
'nice one' becomes 'ta' (both can mean 'thank you!').
'shocker' becomes 'boo' (you might say 'boo!' to shock someone).
'ta'+'boo'='TABOO'
(Other definitions for taboo that I've seen before include "Not to be mentioned" , "Subject to prohibition" , "Ritual prohibition" , "Not to be touched or used, banned for religious reasons" , "(Something) forbidden" .)