Cryptic crossword clue
Solid evidence, initially, for book with characters 5,3,7

The most common and most likely answer to this crossword clue is the 5 plus 3 plus 7 letter word FACTS AND FIGURES

I've seen this crossword clue in Financial Times
F
A
C
T
S
A
N
D
F
I
G
U
R
E
S

Buddy explains!

The straight part
We know that "Solid evidence" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case solid evidence = facts and figures.
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.

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