The most common and most likely answer to this crossword clue is the 9 letter word COMPANION
I've seen this crossword clue in Independent
C
O
M
P
A
N
I
O
N
Buddy explains!
The straight part
We know that "handbook" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case handbook = companion.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 COMPANION mean?
COMPANION noun- A friend, acquaintance, or partner; someone with whom one spends time or keeps company
- A person employed to accompany or travel with another.
- The framework on the quarterdeck of a sailing ship through which daylight entered the cabins below.
- The covering of a hatchway on an upper deck which leads to the companionway; the stairs themselves.
- A knot in whose neighborhood another, specified knot meets every meridian disk.
- A thing or phenomenon that is closely associated with another thing, phenomenon, or person.
- An appended source of media or information, designed to be used in conjunction with and to enhance the main material.
- A celestial object that is associated with another.
- A knight of the lowest rank in certain orders.
- A fellow; a rogue.
- To be a companion to; to attend on; to accompany.
- To qualify as a companion; to make equal.