Great puzzle! It was fun to solve the whole way through.
I found the key realization from the clue "Longest side on a right-angled triangle". The answer is clearly
HYPOTENUSE, which is too long to fit
But, it can be made to work if you
treat the substring "TEN" as the single number "10", putting that number in a single cell of the grid. The idea is to replace a substring with its number.
Solving for other clues the same way gives the answers:
24 grains, or 1/20 of a troy ounce PENNYW8
Acrophobia is the fear of this H8S
An interconnected system NE2RK
Holding of office or land 10URE
Imaginary or unreal N1XIS10T
Intermediary floor MEZZA9
Juvenile felines KIT10S
Longest side on a right-angled triangle HYPO10USE
Pointed tooth CA9
Sheen or lustre SHI9SS
The first to explore unseen territory PI1ER
Unstable oxygen OZ1
and the grid
Finally, we extract by
reading the highlighed letters, which anagram to "SUM" and "ROWS"
This tells us to
find the sum of the numbers in each row, which gives letters:
Finally, this gives the answer
Petit four, from the clues that it's a confectionery and the theme of including a number in the word.
How did you make this? The whole thing seems really really constrained. I can't imagine there's many words to choose from, and the sums seem very restricting along with the intersection requirements.