The most common and most likely answer to this crossword clue is the 6 letter word TIRADE
I've seen this crossword clue in The Sunday Telegraph
T
I
R
A
D
E
More clues leading to the result TIRADE
- Angry outburst from one stuck in traffic
- Blast experienced after impact, just shells going up
- Angry words of one caught in traffic
- One in commerce delivering outburst
- Artist in time for lecture?
- Island dividing traffic gets railing
- Traffic around India produces angry outburst
- String of invectives from traffic island being absorbed
- Stuck in traffic, I rant
- Deal with one in series of 19s
Buddy explains!

The straight part
We know that "Vehement outburst" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case vehement outburst = tirade.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 TIRADE mean?
TIRADE noun- A long, angry or violent speech; a diatribe.
- A section of verse concerning a single theme; a laisse.
- To make a long, angry or violent speech, a tirade.