The most common and most likely answer to this crossword clue is the 8 letter word ZELATRIX
Z
E
L
A
T
R
I
X
Buddy explains!
The straight part
We know that "Overseer of religiously strict conduct" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case overseer of religiously strict conduct = zelatrix.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.