The most common and most likely answer to this crossword clue is the 5 letter word SNARK
S
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Buddy explains!

The straight part
We know that "Imaginary creature" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case imaginary creature = snark.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 SNARK mean?
SNARK noun- Snide remarks.
- To express oneself in a snarky fashion.
- To snort.
- A graph in which every node has three branches, and the edges cannot be coloured in fewer than four colours without two edges of the same colour meeting at a point.
- A fluke or unrepeatable result or detection in an experiment.