The most common and most likely answer to this crossword clue is the 10 plus 4 letter word COLLECTIVE NOUN
C
O
L
L
E
C
T
I
V
E
N
O
U
N
Buddy explains!
The straight part
We know that "One is safe for ducks" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one is safe for ducks = collective noun.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.