The most common and most likely answer to this crossword clue is the 5 plus 5 letter word HORSE THIEF
I've seen this crossword clue in The Guardian
H
O
R
S
E
T
H
I
E
F
Buddy explains!
The straight part
We know that "the western villain" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case the western villain = horse thief.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.