1- Beast of burden (cryptic definition of an animal carrier)
2-Sit in (IT in sin(error)
3-Fell (Double definition) level as in chop down or fell and a hill is also called a fell)
4-Unfeigned Anagram of fudgenine
5- Push the envelope (cryptic definition- something a postman does through a letterbox and doing something seemingly impossible)
6- Vile speaking. (Anagram of elvis) viles + pea king (marrowfat ruler)