The most common and most likely answer to this crossword clue is the 7 plus 6 letter word TRIGGER FINGER
I've seen this crossword clue in The Guardian
T
R
I
G
G
E
R
F
I
N
G
E
R
More clues leading to the result TRIGGER FINGER
Buddy explains!
The straight part
We know that "Gunslinger would have this" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gunslinger would have this = trigger finger.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.