The most common and most likely answer to this crossword clue is the 17 letter word THEÂ MERRYÂ MONARCH
I've seen this crossword clue in The Guardian
T
H
E
Â
M
E
R
R
Y
Â
M
O
N
A
R
C
H
Buddy explains!
The straight part
We know that "Charles the Second" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case charles the second = the merry monarch.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.