The most common and most likely answer to this crossword clue is the 12 letter word OTHERWORLDLY
I've seen this crossword clue in Independent
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Buddy explains!

The straight part
We know that "Like impractical thinker" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case like impractical thinker = otherworldly.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 OTHERWORLDLY mean?
OTHERWORLDLY adjective- Of, concerned with, or preoccupied with a different world than that of the tangible here and now, such as a heavenly, spiritual, or imaginary world.
- Not belonging to the real world; unnatural; odd and unfamiliar.