The most common and most likely answer to this crossword clue is the 4 letter word ORCS
O
R
C
S
Buddy explains!
The straight part
We know that "Whales" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case whales = orcs.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 ORCS mean?
ORCS noun- Any of several large, ferocious sea creatures, now especially the killer whale.
- A mythical evil monstrous humanoid creature, usually quite aggressive and often green.
- A porcine humanoid monster larger than humans, sometimes pink. (found in Japanese pop-culture and called "orc" when imported to the West)