A serious offence in the Orient? That's idiotic (7)
I believe the answer is:
asinine
'idiotic' is the definition.
(thesaurus)
'a serious offence in the orient?' is the wordplay.
'serious offence' becomes 'sin' (I've seen this before).
'the orient?' becomes 'e'.
'a'+'sin'+'in'+'e'='ASININE'
'that's' acts as a link.
(Other definitions for asinine that I've seen before include "Silly like a donkey" , "Idiotic" , "Daft" , "Foolish like donkey" , "In the nature of supposedly stupid animal" .)