The most common and most likely answer to this crossword clue is the 14 letter word ZOROASTRIANISM
I've seen this crossword clue in The Guardian
Z
O
R
O
A
S
T
R
I
A
N
I
S
M
Buddy explains!
The straight part
We know that "old type of religion" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old type of religion = zoroastrianism.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.