The most common and most likely answer to this crossword clue is the 29 letter word REDWTEARFACTARTEFACT/REDWATER
R
E
D
W
T
E
A
R
F
A
C
T
A
R
T
E
F
A
C
T
/
R
E
D
W
A
T
E
R
Buddy explains!
The straight part
We know that "Thing that’s created/serious affliction in the herd" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case thing that’s created/serious affliction in the herd = redwtearfactartefact/redwater.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.