It's forbidden to change a boot (5)
I believe the answer is:
taboo
'it's forbidden' is the definition.
('taboo' can be a synonym of 'forbidden')
'change a boot' is the wordplay.
'change' indicates an anagram.
'a'+'boot'='aboot'
'aboot' anagrammed gives 'TABOO'.
'to' is the link.
(Other definitions for taboo that I've seen before include "A ban or prohibition of superstitious kind" , "Subject to prohibition" , "Prohibited, sacred" , "Prohibited as unclean or holy" , "Ritual prohibition" .)