The most common and most likely answer to this crossword clue is the 5 plus 4 letter word CORAL REEF
I've seen this crossword clue in The Guardian
C
O
R
A
L
R
E
E
F
More clues leading to the result CORAL REEF
- Swimming for clear heart of sea – to see this?
- Recycling of clearer water vital to its survival
- Offshore bank said to be in credit before payment returned?
- Underwater structure needs real force to break
- Essentially searching for a river eel distributed in underwater ecosystem
- Wow! Gangster free after fiddling offshore bank system?
Buddy explains!
The straight part
We know that "Underwater structure" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case underwater structure = coral reef.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.