Hi,Smellyharry,
37dn is a fictional example of the thematic items - the wordplay is a two-letter word meaning “soul” plus the American spelling for a word meaning “cut”, with the last letter (an X) being the extra letter.
27ac is a five-letter word for a type of coffee (often with a chocolate flavour), less M (at the beginning) for money, plus I (at the end) for one. The first letter of this, which is O, is the extra letter. The definition is “tea”.
On the general issue, it’s common for clues to lead to proper nouns that are not in Chambers. Whether the answer to 37dn is sufficiently well-known is open to debate, but it’s given by Bradford in the list of thematic items. As the cells making up this answer are empty (though subject to a thematic treatment) at the end of the puzzle, the answer to 37dn doesn’t breach the statement in the preamble that “the final grid contains only real words”.