Cryptic crossword clue
Old fellow being mature about relationship in book 2,5,7

The most common and most likely answer to this crossword clue is the 2 plus 5 plus 7 letter word OF HUMAN BONDAGE

I've seen this crossword clue in Financial Times
O
F
H
U
M
A
N
B
O
N
D
A
G
E

Buddy explains!

The straight part
We know that "book" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case book = of human bondage.
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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