The most common and most likely answer to this crossword clue is the 8 letter word PLANKTON
P
L
A
N
K
T
O
N
More clues leading to the result PLANKTON
- Tiny creatures of the deep, a hundred on board
- Heavy weight on board, whale food
- King opens shop cooking sea food
- Idiot put on weight: it's the seafood
- Seafood's left in vessel, baked in the middle and not turned over
- Marine creatures, a great many below section of deck
- Seafood board not for turning
- Design keelboat, after vacation, being addicted to marine life
- Board not for turning; it’ll be swallowed by a big fish
- Waterborne creatures arrange untied knot
Buddy explains!

The straight part
We know that "whale food" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case whale food = plankton.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 PLANKTON mean?
PLANKTON noun- Small (often microscopic) organisms that float in the water. (A single organism is known as a plankter.)