The most common and most likely answer to this crossword clue is the 6 letter word GATSBY
I've seen this crossword clue in Financial Times
G
A
T
S
B
Y
More clues leading to the result GATSBY
- Speculator backed by mysterious millionaire
- Great man has guns to hand
- Fictional millionaire just about manages with small change
- Great man, one out and about the bank
- Eponymous hero in book copes with change of character
- Jay, in books, appearing colourful around bank
- Manages to exchange drug for a literary character
- Great Fitzgerald character almost gets by?
- Party man turning up by great man
- Retired speculator close to becoming a millionaire
Buddy explains!

The straight part
We know that "Jay, in books" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case jay, in books = gatsby.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.