The most common and most likely answer to this crossword clue is the 9 plus 10 letter word ROUNDHEAD (CAVALIER)
I've seen this crossword clue in The Guardian
R
O
U
N
D
H
E
A
D
(
C
A
V
A
L
I
E
R
)
Buddy explains!
The straight part
We know that "his" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case his = roundhead (cavalier).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.