The most common and most likely answer to this crossword clue is the 7 plus 7 letter word POISSON D'AVRIL
I've seen this crossword clue in The Times
P
O
I
S
S
O
N
D
'
A
V
R
I
L
Buddy explains!
The straight part
We know that "Cross-channel prank" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cross-channel prank = poisson d'avril.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.