Lip-smacking snack? (7,8)
I believe the answer is:
Here is my best explanation:
'lip-smacking snack?' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.
'smacking snack?' is the wordplay.
'smacking' becomes 'knuckle' (I can't justify this - if you can you should give a lot more credence to this answer).
'snack?' becomes 'sandwich' (I've seen this before).
(Other definitions for knuckle sandwich that I've seen before include "bunch", "Buffet", "hit", "Punch in mug", "piece of pork between slices of bread", "Bunch of fives" and "Slug".)