Ban cheers and a more hostile expression (5)
I believe the answer is:
taboo
'ban' is the definition.
(I've seen this before)
'cheers and a more hostile expression' is the wordplay.
'cheers' becomes 'ta' (I've seen this in other clues).
'and' says to put letters next to each other.
'a more hostile expression' becomes 'boo' (I can't explain this - if you can you should believe this answer much more).
'ta'+'boo'='TABOO'
Can you help me to learn more?
(Other definitions for taboo that I've seen before include "(Irrational) prohibition" , "Prohibited by social custom" , "Ritually forbidden" , "Beyond the pale" , "A ban or prohibition of superstitious kind" .)