The most common and most likely answer to this crossword clue is the 6 letter word MYOSIS
M
Y
O
S
I
S
Buddy explains!
The straight part
We know that "overreaction of pupils" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case overreaction of pupils = myosis.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 MYOSIS mean?
MYOSIS noun- Contraction of the pupil of the eye.