The most common and most likely answer to this crossword clue is the 5 letter word HARAM
I've seen this crossword clue in The Times
H
A
R
A
M
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The straight part
We know that "forbidden by religious law" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case forbidden by religious law = haram.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 HARAM mean?
HARAM noun- A sin.
- (fiqh) Forbidden by Islam: unlawful, sinful.