But a change will be forbidden (4)
I believe the answer is:
tabu
'forbidden' is the definition.
(synonyms)
'but a change' is the wordplay.
'change' is an anagram indicator.
'but'+'a'='buta'
'buta' is an anagram of 'TABU'.
'will be' acts as a link.
(Other definitions for tabu that I've seen before include "Disapproved of" , "Forbidden (Alt sp.)" , "Ban resulting from social custom" , "delicate matter?" , "banned" .)