The most common and most likely answer to this crossword clue is the 5 letter word TERCE
T
E
R
C
E
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The straight part
We know that "Hour of prayer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case hour of prayer = terce.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 TERCE mean?
TERCE noun- The third hour of daylight (about 9 am).
- The service appointed for this hour.
- A widow's right, where she has no conventional provision, to a liferent of a third of the husband's heritable property.