The most common and most likely answer to this crossword clue is the 12 letter word SPRINGÂ CLEAN
I've seen this crossword clue in The Guardian
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Buddy explains!
The straight part
We know that "annual purification" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case annual purification = spring clean.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 SPRINGÂ CLEAN mean?
SPRINGÂ CLEAN verb- To carry out spring-cleaning on (a room, a house, etc).
- To engage in spring-cleaning.