The most common and most likely answer to this crossword clue is the 6 letter word NOYADE
I've seen this crossword clue in The Times
N
O
Y
A
D
E
Buddy explains!
The straight part
We know that "execution by drowning" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case execution by drowning = noyade.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 NOYADE mean?
NOYADE noun- A murder by drowning, especially one of those carried out during the French Reign of Terror.
- To murder by drowning, especially during the French Reign of Terror.