The most common and most likely answer to this crossword clue is the 5 letter word TILAK
T
I
L
A
K
Buddy explains!
The straight part
We know that "Religious decoration once" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case religious decoration once = tilak.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.
What does TILAK mean?
TILAK noun- A mark or symbol worn on the forehead by Hindus, ornamentally or as an indication of status.