The most common and most likely answer to this crossword clue is the 6 letter word VOWING
I've seen this crossword clue in Independent
V
O
W
I
N
G
More clues leading to the result VOWING
Buddy explains!

The straight part
We know that "Swearing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case swearing = vowing.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 VOWING mean?
VOWING verb- To make a vow; to promise.
- To make a vow regarding (something).
- To declare publicly that one has made a vow, usually to show one's determination or to announce an act of retaliation.