The most common and most likely answer to this crossword clue is the 8 letter word HIGH-SET
I've seen this crossword clue in Financial Times
H
I
G
H
-
S
E
T
More clues leading to the result HIGH-SET
Buddy explains!
The straight part
We know that "in the upper strata" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in the upper strata = high-set.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.