The most common and most likely answer to this crossword clue is the 9 letter word IMMERSION
I've seen this crossword clue in The Guardian
I
M
M
E
R
S
I
O
N
Buddy explains!
The straight part
We know that "Baptism" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case baptism = immersion.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 IMMERSION mean?
IMMERSION noun- The act of immersing or the condition of being immersed.
- An immersion heater.
- A smooth map whose differential is everywhere injective, related to the mathematical concept of an embedding.
- The disappearance of a celestial body, by passing either behind another, as in the occultation of a star, or into its shadow, as in the eclipse of a satellite.
- A form of foreign-language teaching where the language is used intensively to teach other subjects to a student.