Meat-filled bun for beach picnic (8)
I believe the answer is:
clambake
'beach picnic' is the definition.
(I've seen this in another clue)
'meat-filled bun' is the wordplay.
'meat' becomes 'lamb' (lamb is a kind of meat).
'filled' is an insertion indicator.
'bun' becomes 'cake' (I can't explain this - if you can you should give a lot more credence to this answer).
'lamb' going within 'cake' is 'CLAMBAKE'.
'for' is the link.
Can you help me to learn more?
(Other definitions for clambake that I've seen before include "Picnic over the pond" , "Picnic, American style" , "Picnic in America" , "Fishy equivalent of barbecue" , "For outdoors" .)