The most common and most likely answer to this crossword clue is the 12 letter word HIPPOPOTAMUS
I've seen this crossword clue in The Times
H
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More clues leading to the result HIPPOPOTAMUS
- I put shampoo liberally around head of poodle — muddy animal?
- Wallower in trendy popular music, returning to entertain endlessly
- Creature this puma moved across river twice
- Big grey creature, one with power to move suddenly, surrounded by worried mahouts
- Water animal with a hump — it’s moving round river repeatedly
- Animals in river bank charm endlessly
- Animal in river basin, with endless delight
Buddy explains!

The straight part
We know that "Wallower" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case wallower = hippopotamus.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 HIPPOPOTAMUS mean?
HIPPOPOTAMUS noun- A large, semi-aquatic, herbivorous (plant-eating) African mammal (Hippopotamus amphibius)