The most common and most likely answer to this crossword clue is the 9 letter word FANTASIST
I've seen this crossword clue in The Guardian
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More clues leading to the result FANTASIST
- Female worker getting help, we hear, for a Walter Mitty type?
- Punching the puncher, soldier as Walter Mitty?
- One with a dream lover posed from behind first
- Duke describes worker as a Walter Mitty character
- Deluded sort of enthusiast requiring no change in Manx event
- Soldier, when engaged in writing, is a dreamer
- For example, Tolkien enthusiast is overwhelmed by endless choice
Buddy explains!

The straight part
We know that "Walter Mitty" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case walter mitty = fantasist.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 FANTASIST mean?
FANTASIST noun- One who creates fantasies.
- One living in a fantasy world.
- A writer who writes in the fantasy style.