The most common and most likely answer to this crossword clue is the 6 letter word SURELY
I've seen this crossword clue in Financial Times
S
U
R
E
L
Y
Buddy explains!

The straight part
We know that "with confidence" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case with confidence = surely.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 SURELY mean?
SURELY adverb- Without fail.
- Certainly, undoubtedly.
- With confidence.