Legally permissible to start late, but offensive (5)
I believe the answer is:
awful
'offensive' is the definition.
(thesaurus)
'legally permissible to start late' is the wordplay.
'legally permissible' becomes 'lawful' (I've seen this before).
'to start late' means to remove the first letter (the word starts from its second letter).
'lawful' with its first letter taken off is 'AWFUL'.
'but' is the link.
(Other definitions for awful that I've seen before include "rubbish" , "Horrible" , "pants" , "Very poor" , "Appalling, terrible" .)