The most common and most likely answer to this crossword clue is the 12 letter word IMPONDERABLE
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More clues leading to the result IMPONDERABLE
Buddy explains!

The straight part
We know that "Elusive" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case elusive = imponderable.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 IMPONDERABLE mean?
IMPONDERABLE noun- An imponderable substance or body; specifically, in the plural, a name formerly applied to heat, light, electricity, and magnetism.
- An imponderable question.
- Not ponderable; without sensible or appreciable weight; incapable of being weighed.
- Difficult or impossible to comprehend or evaluate.