The most common and most likely answer to this crossword clue is the 9 letter word POLO-NECK
I've seen this crossword clue in The Times
P
O
L
O
-
N
E
C
K
More clues leading to the result POLO-NECK
Buddy explains!

The straight part
We know that "Jumper" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case jumper = polo-neck.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 POLO-NECK mean?
POLO-NECK noun- A high collar covering the neck; a turtleneck.
- A light type of shirt with such a collar; a turtleneck.