After cheers, negative utterance forbidden (5)
I believe the answer is:
taboo
'forbidden' is the definition.
(synonyms)
'after cheers negative utterance' is the wordplay.
'after' means one lot of letters go next to another.
'cheers' becomes 'ta' (both can informally mean thank you).
'negative utterance' becomes 'boo' (as in a crowd booing).
'ta'+'boo' is 'TABOO'.
(Other definitions for taboo that I've seen before include "A ban or prohibition of superstitious kind" , "Socially prohibited" , "Prohibited, sacred" , "(Irrational) prohibition" , "Prohibited by social custom" .)