The most common and most likely answer to this crossword clue is the 5 letter word SMEWS
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The straight part
We know that "Diving ducks" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case diving ducks = smews.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 SMEWS mean?
SMEWS noun- A small compact diving duck, Mergus albellus, that breeds in the northern taiga of Europe and Asia and winters on sheltered coasts or inland lakes.