The most common and most likely answer to this crossword clue is the 5 plus 7 letter word DEATH PENALTY
I've seen this crossword clue in The Times
D
E
A
T
H
P
E
N
A
L
T
Y
Buddy explains!
The straight part
We know that "Severe punishment" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case severe punishment = death penalty.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.