The most common and most likely answer to this crossword clue is the 5 letter word CAULK
I've seen this crossword clue in The Guardian
C
A
U
L
K
Buddy explains!
The straight part
We know that "Stop boat leaking" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case stop boat leaking = caulk.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 CAULK mean?
CAULK noun- A pointed projection on a horseshoe to prevent it slipping.
- A spike on the sole of a boot to prevent slipping, particularly used in logging
- Caulking.
- A composition of vehicle and pigment used at ambient temperatures for filling/sealing joints or junctures, that remains elastic for an extended period of time after application.
- To drive oakum into the seams of a ship's wooden deck or hull to make it watertight.
- To apply caulking to joints, cracks, or a juncture of different materials.
- Fuck