The most common and most likely answer to this crossword clue is the 7 letter word FORTIFY
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More clues leading to the result FORTIFY
Buddy explains!

The straight part
We know that "confirm" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case confirm = fortify.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 FORTIFY mean?
FORTIFY verb- To increase the defenses of; to strengthen and secure by military works; to render defensible against an attack by hostile forces.
- To impart strength or vigor to.
- To add spirits to wine to increase the alcohol content.
- To increase the nutritional value of food by adding ingredients.