The most common and most likely answer to this crossword clue is the 2 plus 3 plus 6 letter word IN THE SADDLE
I've seen this crossword clue in The Daily Telegraph
I
N
T
H
E
S
A
D
D
L
E
More clues leading to the result IN THE SADDLE
Buddy explains!
The straight part
We know that "Holding the reins of office?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case holding the reins of office? = in the saddle.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.