The most common and most likely answer to this crossword clue is the 12 letter word PARATROOPERS
I've seen this crossword clue in The Guardian
P
A
R
A
T
R
O
O
P
E
R
S
Buddy explains!
The straight part
We know that "Dropping unit from sky" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dropping unit from sky = paratroopers.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 PARATROOPERS mean?
PARATROOPERS noun- A type of soldier who is trained to enter combat zones by parachuting from aircraft.