The most common and most likely answer to this crossword clue is the 7 letter word NERVIER
I've seen this crossword clue in The Guardian
N
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R
V
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More clues leading to the result NERVIER
Buddy explains!

The straight part
We know that "with less assurance" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case with less assurance = nervier.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 NERVIER mean?
NERVIER adjective- Having nerve; bold; brazen.
- Feeling nervous, anxious or agitated.
- Strong; sinewy.
- Jittery; having unwanted signal characteristics.