Exceptionally bad sin — he gets cast into the wilderness? (8)
I believe the answer is:
banished
'cast into the wilderness?' is the definition.
(to banish is to cast out)
'exceptionally bad sin he' is the wordplay.
'exceptionally' indicates an anagram.
'bad'+'sin'+'he'='badsinhe'
'badsinhe' anagrammed gives 'BANISHED'.
'gets' acts as a link.
(Other definitions for banished that I've seen before include "Sent into exile" , "Condemned to exile" , "Exiled" , "excluded?" .)