The most common and most likely answer to this crossword clue is the 10 letter word WATCHMAKER
I've seen this crossword clue in The Daily Telegraph
W
A
T
C
H
M
A
K
E
R
More clues leading to the result WATCHMAKER
Buddy explains!

The straight part
We know that "Horologist" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case horologist = watchmaker.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.
What does WATCHMAKER mean?
WATCHMAKER noun- A person who repairs (and originally made) watches.