The most common and most likely answer to this crossword clue is the 6 letter word VOTIVE
I've seen this crossword clue in Independent
V
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More clues leading to the result VOTIVE
Buddy explains!
The straight part
We know that "undertaken to fulfil promise" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case undertaken to fulfil promise = votive.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 VOTIVE mean?
VOTIVE noun- A hymn or chant dedicated to a particular saint, or to the Virgin Mary
- Dedicated or given in fulfillment of a vow or pledge
- Of, expressing or symbolizing a vow. Often used to describe thick cylindrical candles found in many churches, lit when making a private vow or asking a private intention.