The most common and most likely answer to this crossword clue is the 10 plus 5 letter word PERFORMING FLEAS
I've seen this crossword clue in Financial Times
P
E
R
F
O
R
M
I
N
G
F
L
E
A
S
Buddy explains!
The straight part
We know that "Show-jumpers" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case show-jumpers = performing fleas.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.