The most common and most likely answer to this crossword clue is the 5 plus 1 letter word OATH} –
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The straight part
We know that "promise" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case promise = oath} –.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 OATH} – mean?
OATH} – noun- A solemn pledge or promise, appealing to a deity, a ruler, or another entity (not necessarily present) to attest to the truth of a statement or sincerity of one's desire to fulfill a contract or promise.
- A statement or promise which is strengthened (affirmed) by such a pledge.
- A light, irreverent or insulting appeal to a deity or other entity.
- A curse, a curse word.
- To pledge.
- A solemn pledge or promise, appealing to a deity, a ruler, or another entity (not necessarily present) to attest to the truth of a statement or sincerity of one's desire to fulfill a contract or promise.
- A statement or promise which is strengthened (affirmed) by such a pledge.
- A light, irreverent or insulting appeal to a deity or other entity.
- A curse, a curse word.
- To pledge.