The most common and most likely answer to this crossword clue is the 10 letter word TERRACOTTA
I've seen this crossword clue in The Guardian
T
E
R
R
A
C
O
T
T
A
More clues leading to the result TERRACOTTA
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 = terracotta.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 TERRACOTTA mean?
TERRACOTTA noun- A hard red-brown unglazed earthenware, used for pottery and building construction.
- A reddish brown colour, like that of terra cotta.
- Of the colour of terracotta.