The most common and most likely answer to this crossword clue is the 4 plus 8 letter word LONG SENTENCE
I've seen this crossword clue in The Daily Telegraph
L
O
N
G
S
E
N
T
E
N
C
E
Buddy explains!
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.