The most common and most likely answer to this crossword clue is the 10 letter word BACKWATERS
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Buddy explains!

The straight part
We know that "lagoons" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lagoons = backwaters.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 BACKWATERS mean?
BACKWATERS noun- The water held back by a dam or other obstruction
- A remote place; somewhere that remains unaffected by new events, progresses, ideas, etc.
- A rowing stroke in which the oar is pushed forward to stop the boat; see back water
- To row or paddle a backwater stroke.
- To vacillate on a long-held position.