The most common and most likely answer to this crossword clue is the 7 letter word UPCHUCK
I've seen this crossword clue in Financial Times
U
P
C
H
U
C
K
Buddy explains!
The straight part
We know that "React to bug in US" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case react to bug in us = upchuck.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 UPCHUCK mean?
UPCHUCK noun- Vomit.
- To vomit.