The most common and most likely answer to this crossword clue is the 10 letter word DAYDREAMER
I've seen this crossword clue in The Times
D
A
Y
D
R
E
A
M
E
R
More clues leading to the result DAYDREAMER
- Medic keeping New Year made one in a brown study
- Stargazer adjusted ready-made refractor’s tip
- Noblewoman about ready to reform Republican fantasist
- One going off to moon with odd ready-made lander finally
- One’s imagining Romeo with shoddy readymade clothing
- Fantasist armed and ready for manoeuvres
- Train ready-made radical fantasist
- Titled woman about ready to reform king? Idealist
- Fantasist could be armed and ready …
- Does one not enjoy night vision?
Buddy explains!
The straight part
We know that "one in a brown study" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one in a brown study = daydreamer.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.