The most common and most likely answer to this crossword clue is the 8 letter word INFINITY
I've seen this crossword clue in The Daily Telegraph
I
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Buddy explains!
The straight part
We know that "vastness" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case vastness = infinity.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 INFINITY mean?
INFINITY noun- Endlessness, unlimitedness, absence of a beginning, end or limits to size.
- A number that has an infinite numerical value that cannot be counted.
- An idealised point which is said to be approached by sequences of values whose magnitudes increase without bound.
- A number which is very large compared to some characteristic number. For example, in optics, an object which is much further away than the focal length of a lens is said to be "at infinity", as the distance of the image from the lens varies very little as the distance increases further.
- The symbol ∞.