The most common and most likely answer to this crossword clue is the 7 plus 8 letter word KNUCKLE SANDWICH
I've seen this crossword clue in Independent
K
N
U
C
K
L
E
S
A
N
D
W
I
C
H
Buddy explains!
The straight part
We know that "Lip-smacking snack" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lip-smacking snack = knuckle sandwich.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.