Forbidden, thanks to an expression of disapproval (5)
I believe the answer is:
taboo
'forbidden' is the definition.
(synonyms)
'thanks to an expression of disapproval' is the wordplay.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'to an expression of disapproval' becomes 'boo' (as in a crowd booing).
'ta'+'boo'='TABOO'
(Other definitions for taboo that I've seen before include "Not allowed, forbidden, proscribed" , "Subject to prohibition" , "Beyond the pale" , "A ban or prohibition of superstitious kind" , "Inhibition based on social custom" .)