The most common and most likely answer to this crossword clue is the 6 letter word LUGGER
I've seen this crossword clue in The Guardian
L
U
G
G
E
R
Buddy explains!
The straight part
We know that "Sounds like a tugboat, but isn’t" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sounds like a tugboat, but isn’t = lugger.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 LUGGER mean?
LUGGER noun- That which lugs in either literal or figurative senses.
- One who lugs, especially one whose job entails pulling or moving heavy objects.
- A conman.
- A person hired by a gambling establishment to locate potential customers and bring them in.
- A small vessel having two or three masts, and a running bowsprit, and carrying lugsails.
- An Indian falcon (Falco jugger), similar to the European lanner and the American prairie falcon.