The most common and most likely answer to this crossword clue is the 3 plus 7 letter word HAD KITTENS
I've seen this crossword clue in The Sunday Telegraph
H
A
D
K
I
T
T
E
N
S
Buddy explains!
The straight part
We know that "Dropped litter/reacted with alarm" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dropped litter/reacted with alarm = had kittens.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.