The most common and most likely answer to this crossword clue is the 6 letter word ROCKET
I've seen this crossword clue in The Times
R
O
C
K
E
T
Buddy explains!

The straight part
We know that "Severe reprimand/firework" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case severe reprimand/firework = rocket.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 ROCKET mean?
ROCKET noun- A rocket engine.
- A non-guided missile propelled by a rocket engine.
- A vehicle propelled by a rocket engine.
- A rocket propelled firework, a skyrocket
- An ace (the playing card).
- An angry communication (such as a letter or telegram) to a subordinate.
- A blunt lance head used in jousting.
- Something that shoots high in the air.
- A stupid or crazy person.
- To accelerate swiftly and powerfully
- To fly vertically
- To rise or soar rapidly
- To carry something in a rocket
- To attack something with rockets
- The leaf vegetable Eruca sativa or Eruca vesicaria.
- Rocket larkspur (Consolida regalis)