"Cheers!” and "Shame!” said to be unacceptable (4)
I believe the answer is:
tabu
'unacceptable' is the definition.
(another spelling of 'taboo')
''cheers' and 'shame' said' is the wordplay.
''cheers'' becomes 'ta' (both can mean thank you).
'and' means one lot of letters go next to another.
''shame'' becomes 'boo' (expressions of displeasure or disgust).
'said' indicates a 'sounds like' (homophone) clue.
'boo' sounds like 'bu'.
'ta'+'bu'='TABU'
'to be' is the link.
(Other definitions for tabu that I've seen before include "Prohibited practice" , "delicate matter?" , "Ban resulting from social custom" , "Disapproved of" , "Unacceptable" .)