The most common and most likely answer to this crossword clue is the 8 plus 7 letter word JEHOVAHS WITNESS
I've seen this crossword clue in Independent
J
E
H
O
V
A
H
S
W
I
T
N
E
S
S
More clues leading to the result JEHOVAHS WITNESS
Buddy explains!

The straight part
We know that "sectarian" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sectarian = jehovahs witness.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.