The most common and most likely answer to this crossword clue is the 6 letter word MERMAN
M
E
R
M
A
N
Buddy explains!
The straight part
We know that "legless bloke found in sea" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case legless bloke found in sea = merman.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 MERMAN mean?
MERMAN noun- A legendary creature, human male from the waist up, fishlike from the waist down.