The most common and most likely answer to this crossword clue is the 11 plus 3 letter word HARD-BOILED EGG
I've seen this crossword clue in Independent
H
A
R
D
-
B
O
I
L
E
D
E
G
G
Buddy explains!
The straight part
We know that "snack" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case snack = hard-boiled egg.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.