The most common and most likely answer to this crossword clue is the 7 letter word MUSSELS
I've seen this crossword clue in The Sunday Telegraph
M
U
S
S
E
L
S
More clues leading to the result MUSSELS
Buddy explains!

The straight part
We know that "Seafood" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case seafood = mussels.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.
What does MUSSELS mean?
MUSSELS noun- Any of several groups of bivalve shellfish with elongated, asymmetrical shells