The most common and most likely answer to this crossword clue is the 9 letter word BAPTIZING
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Buddy explains!

The straight part
We know that "christening" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case christening = baptizing.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 BAPTIZING mean?
BAPTIZING verb- To perform the sacrament of baptism by sprinkling or pouring water over someone or immersing them in water.
- To dedicate or christen.
- Of rum, brandy, or any other spirits, to dilute with water.
- To ensure proper burning of a joint by moistening the exterior with saliva.
- Baptism