The most common and most likely answer to this crossword clue is the 8 letter word ROMANISE
I've seen this crossword clue in The Daily Telegraph
R
O
M
A
N
I
S
E
More clues leading to the result ROMANISE
Buddy explains!

The straight part
We know that "to become a Catholic" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case to become a catholic = romanise.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 ROMANISE mean?
ROMANISE verb- To put letters or words written in another writing system into the Latin (Roman) alphabet.
- (usually capitalized) To bring under the authority or influence of Rome.
- (usually capitalized) To make or become Roman in character or style.
- (usually capitalized) To make or become Roman Catholic in religion (by conversion), character or style.
- To fill with Latin words or idioms.