The most common and most likely answer to this crossword clue is the 8 letter word NOUMENON
I've seen this crossword clue in Financial Times
N
O
U
M
E
N
O
N
Buddy explains!
The straight part
We know that "something not sensed" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case something not sensed = noumenon.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 NOUMENON mean?
NOUMENON noun- A thing as it is independent of any conceptualization or perception by the human mind, postulated by practical reason but existing in a condition which is in principle unknowable and unexperienceable.