The most common and most likely answer to this crossword clue is the 5 letter word KRILL
I've seen this crossword clue in The Times
K
R
I
L
L
More clues leading to the result KRILL
- Plankton: prey rorquals principally swallowed
- Big swimmers gulp it while away crossing river
- Prey dipped into by rorqual’s mouth, main diet?
- Whale food makes a couple of kings sick
- Aquatic creatures from river involved in slaughter
- Crustaceans cook finally found on stream
- Marine creatures run into prey
- 10 preserving sample of rubbish in ice
Buddy explains!

The straight part
We know that "Plankton" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case plankton = krill.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 KRILL mean?
KRILL noun- Any of several small marine crustacean species of plankton in the order Euphausiacea in the class Malacostraca.