The most common and most likely answer to this crossword clue is the 5 letter word VOWEL
I've seen this crossword clue in Financial Times
V
O
W
E
L
More clues leading to the result VOWEL
- I, for one, love dancing around wife
- Maybe I swear to overthrow the French
- I for one love jerks, wife admitted
- I perhaps swear especially loudly for starters
- Facetiously puts one – and others like it – in order
- I, for example, have unpaid bill that five pounds should cover?
- I, for one, love rum infused with whiskey
- Possibly a solemn promise by English learner
- Maybe I am behind first of lapwings, flock of geese going overhead?
- This could be a pledge by the Spanish
Buddy explains!

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 VOWEL mean?
VOWEL noun- A sound produced by the vocal cords with relatively little restriction of the oral cavity, forming the prominent sound of a syllable.
- A letter representing the sound of vowel; in English, the vowels are a, e, i, o, u, and sometimes y.
- To add vowel points to a consonantal script (e.g. niqqud in Hebrew or harakat in Arabic)