The most common and most likely answer to this crossword clue is the 10 letter word PARENTHOOD
I've seen this crossword clue in The Sunday Telegraph
P
A
R
E
N
T
H
O
O
D
More clues leading to the result PARENTHOOD
Buddy explains!

The straight part
We know that "it’s a state he’s in" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it’s a state he’s in = parenthood.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 PARENTHOOD mean?
PARENTHOOD noun- The state of being a parent