The most common and most likely answer to this crossword clue is the 8 letter word SCALABLE
I've seen this crossword clue in The Guardian
S
C
A
L
A
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More clues leading to the result SCALABLE
Buddy explains!

The straight part
We know that "not insurmountable" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not insurmountable = scalable.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 SCALABLE mean?
SCALABLE adjective- Capable of being climbed.
- Able to be changed in scale; resizeable.
- , (logistics), Able to greatly increase in capacity, with relative ease.