The most common and most likely answer to this crossword clue is the 7 letter word HARDPAN
I've seen this crossword clue in Independent
H
A
R
D
P
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Buddy explains!
The straight part
We know that "subsoil layer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case subsoil layer = hardpan.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 HARDPAN mean?
HARDPAN noun- A former lake, especially a lake that has dried and habitually remains dry, opposed to playas.
- A distinct layer of soil that is largely impervious to water.