The most common and most likely answer to this crossword clue is the 5 letter word AMEBA
A
M
E
B
A
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The straight part
We know that "Protean US creature" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case protean us creature = ameba.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 AMEBA mean?
AMEBA noun- A member of the genus Amoeba of unicellular protozoa that moves by means of temporary projections called pseudopodia.
- The graph of the real part of the logarithms of a polynomial equation in complex numbers.
- An asexual.