Expressions of gratitude and disapproval are forbidden (5)
I believe the answer is:
taboo
'are forbidden' is the definition.
(synonyms)
'expressions of gratitude and disapproval' is the wordplay.
'expressions of gratitude' becomes 'ta' (I can't justify this - if you can you should believe this answer much more).
'and' is a charade indicator (letters next to each other).
'disapproval' becomes 'boo' (as in a crowd booing).
'ta'+'boo'='TABOO'
Can you help me to learn more?
(Other definitions for taboo that I've seen before include "Socially prohibited" , "Prohibited, sacred" , "Socially forbidden" , "Ritual prohibition" , "Prohibited as unclean or holy" .)