The most common and most likely answer to this crossword clue is the 6 letter word GUFFAW
I've seen this crossword clue in Financial Times
G
U
F
F
A
W
Buddy explains!

The straight part
We know that "Laugh loudly" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case laugh loudly = guffaw.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 GUFFAW mean?
GUFFAW noun- A boisterous laugh.
- To laugh boisterously.