The most common and most likely answer to this crossword clue is the 7 letter word FALLOUT
I've seen this crossword clue in The Sunday Telegraph
F
A
L
L
O
U
T
More clues leading to the result FALLOUT
Buddy explains!
The straight part
We know that "Negative side effect" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case negative side effect = fallout.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 FALLOUT mean?
FALLOUT noun- The event of small airborne particles falling to the ground in significant quantities as a result of major industrial activity, volcano eruption, sandstorm, nuclear explosion, etc.
- The particles themselves.
- A negative side effect; an undesirable or unexpected consequence.
- A declined offer in a sales transaction when acceptance was presumed.
- The person who declines such an offer.
- An impromptu guest used to fill in for another guest spot who is a no show or who has cancelled last minute