The most common and most likely answer to this crossword clue is the 9 letter word FLOWSTONE
F
L
O
W
S
T
O
N
E
Buddy explains!
The straight part
We know that "cave structure" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cave structure = flowstone.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 FLOWSTONE mean?
FLOWSTONE noun- A secondary layered mineral deposit of calcite or other mineral, formed by water flowing down the walls and along the floor of a cave.