The most common and most likely answer to this crossword clue is the 10 letter word CHIMERICAL
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The straight part
We know that "Visionary" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case visionary = chimerical.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.
Crossword Answer definitions
CHIMERICAL adjective- Of or pertaining to a chimera.
- Being a figment of the imagination; fantastic (in the archaic sense).
- Inherently fantastic; wildly fanciful.
- Resulting from the expression of two or more genes that originally coded for separate proteins.