The most common and most likely answer to this crossword clue is the 5 plus 5 plus 5 letter word GREAT WHITE SHARK
I've seen this crossword clue in Financial Times
G
R
E
A
T
W
H
I
T
E
S
H
A
R
K
More clues leading to the result GREAT WHITE SHARK
- Swimmer ultimately floundering: where is that wretched rescue vessel?
- Hawk regrets a hit? That’s odd for an aggressive creature
- Predacious type in water rakes thigh — beat it!
- 18a & 21a High water, and streak materialising … that’s very scary!
- Remarkable race, perhaps, dashes in the middle; boat and fish
- I gawk at thresher at sea, a predatory creature
- Craft takes high water around edge of reef, a hidden danger at sea
Buddy explains!
The straight part
We know that "aggressive creature" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case aggressive creature = great white shark.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.