Brownie possibly eating meat? It’s Uncle Sam’s picnic (8)
I believe the answer is:
clambake
'uncle sam's picnic' is the definition.
(US term for a picnic)
'brownie possibly eating meat?' is the wordplay.
'brownie possibly' becomes 'cake' (brownie is a kind of cake).
'eating' means one lot of letters goes inside another.
'meat?' becomes 'lamb' (lamb is a kind of meat).
'cake' enclosing 'lamb' is 'CLAMBAKE'.
'it's' is the link.
(Other definitions for clambake that I've seen before include "Picnic over the pond" , "Outdoor gathering to eat seafood (US)" , "Picnic in America" , "For outdoors" , "Fishy equivalent of barbecue" .)