The most common and most likely answer to this crossword clue is the 6 plus 4 letter word DEADLY BLOW
I've seen this crossword clue in The Guardian
D
E
A
D
L
Y
B
L
O
W
More clues leading to the result DEADLY BLOW
Buddy explains!
The straight part
We know that "Strong punch that would make you croak" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case strong punch that would make you croak = deadly blow.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.