The most common and most likely answer to this crossword clue is the 5 plus 4 plus 3 letter word RIGHT HAND MAN
I've seen this crossword clue in The Guardian
R
I
G
H
T
H
A
N
D
M
A
N
Buddy explains!
The straight part
We know that "Presumably he can be relied upon for dexterity" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case presumably he can be relied upon for dexterity = right hand man.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.