The most common and most likely answer to this crossword clue is the 8 letter word GLEESOME
I've seen this crossword clue in The Times
G
L
E
E
S
O
M
E
More clues leading to the result GLEESOME
Buddy explains!

The straight part
We know that "full of merriment" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case full of merriment = gleesome.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 GLEESOME mean?
GLEESOME adjective- Characterised or marked by glee; gleeful; joyous.