The most common and most likely answer to this crossword clue is the 11 plus 4 letter word SEDIMENTARY ROCK
S
E
D
I
M
E
N
T
A
R
Y
R
O
C
K
Buddy explains!
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.