The most common and most likely answer to this crossword clue is the 7 plus 5 letter word FULLERS EARTH
I've seen this crossword clue in The Times
F
U
L
L
E
R
S
E
A
R
T
H
More clues leading to the result FULLERS EARTH
Buddy explains!
The straight part
We know that "clay" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case clay = fullers earth.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.