The most common and most likely answer to this crossword clue is the 6 plus 3 plus 6 letter word SLINGS AND ARROWS
S
L
I
N
G
S
A
N
D
A
R
R
O
W
S
Buddy explains!
The straight part
We know that "Adverse reactions" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case adverse reactions = slings and arrows.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.