The most common and most likely answer to this crossword clue is the 5 letter word SPURT
I've seen this crossword clue in The Guardian
S
P
U
R
T
Buddy explains!

The straight part
We know that "jet" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case jet = spurt.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 SPURT mean?
SPURT noun- A brief gush, as of liquid spurting from an orifice or a cut/wound.
- Ejaculation of semen.
- A shoot; a bud.
- To cause to gush out suddenly or violently in a stream or jet.
- To rush from a confined place in a small stream or jet.
- A moment, a short period of time.
- A sudden brief burst of, or increase in, speed, effort, activity, emotion or development.
- To make a strong effort for a short period of time.