The most common and most likely answer to this crossword clue is the 13 letter word TRANSMUTATION
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More clues leading to the result TRANSMUTATION
Buddy explains!

The straight part
We know that "alchemy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case alchemy = transmutation.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does TRANSMUTATION mean?
TRANSMUTATION noun- Change, alteration.
- The conversion of one thing into something else; transformation.
- Specifically, the supposed transformation of one element into another, especially of a base metal into gold.
- The actual transformation of one element into another by a nuclear reaction.