The most common and most likely answer to this crossword clue is the 9 letter word UNUTTERED
I've seen this crossword clue in The Times
U
N
U
T
T
E
R
E
D
More clues leading to the result UNUTTERED
Buddy explains!

The straight part
We know that "kept to himself" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case kept to himself = unuttered.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 UNUTTERED mean?
UNUTTERED adjective- Not uttered; unspoken.