Using a boot is forbidden (5)
I believe the answer is:
taboo
'is forbidden' is the definition.
(synonyms)
'using a boot' is the wordplay.
'using' is an anagram indicator (the letters used in a different way).
'a'+'boot'='aboot'
'aboot' with letters rearranged gives 'TABOO'.
(Other definitions for taboo that I've seen before include "Social prohibition" , "By religion or custom, not to be done or touched" , "Beyond the pale" , "Inhibition based on social custom" , "Socially forbidden" .)