The most common and most likely answer to this crossword clue is the 15 letter word ANTICLERICALISM
I've seen this crossword clue in The Daily Telegraph
A
N
T
I
C
L
E
R
I
C
A
L
I
S
M
Buddy explains!
The straight part
We know that "Being opposed to any priest" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case being opposed to any priest = anticlericalism.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.