The most common and most likely answer to this crossword clue is the 4 letter word PROA
I've seen this crossword clue in The Daily Telegraph
P
R
O
A
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.
What does PROA mean?
PROA noun- A sailing vessel found in the waters of Micronesia and Indonesia; it has a single, large outrigger and a triangular sail.