The most common and most likely answer to this crossword clue is the 16 letter word JONATHANÂ EDWARDS
I've seen this crossword clue in The Guardian
J
O
N
A
T
H
A
N
Â
E
D
W
A
R
D
S
Buddy explains!

The straight part
We know that "old jumper" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old jumper = jonathan edwards.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.