Hi, dobbo,
This type of clue is technically known as a composite (or compound) anagram, abbreviated as “comp. anag.”. The word “this” stands for the answer to the clue, and “I trash” plus “scone” is an anagram of “Cornish tea’s”. Azed is a great fan of comp. anag. clues, and the monthly prize-winning clues are often of this form.
This clue could also be an example of the “& lit.” clue, where the clue as a whole serves as both definition and wordplay.