The most common and most likely answer to this crossword clue is the 11 letter word HENOTHEISTS
H
E
N
O
T
H
E
I
S
T
S
Buddy explains!
The straight part
We know that "Believers in just one supreme god" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case believers in just one supreme god = henotheists.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.