The most common and most likely answer to this crossword clue is the 6 plus 6 letter word VAPOUR TRAILS
I've seen this crossword clue in The Daily Telegraph
V
A
P
O
U
R
T
R
A
I
L
S
Buddy explains!
The straight part
We know that "Jumbo might show these" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case jumbo might show these = vapour trails.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.