The most common and most likely answer to this crossword clue is the 6 letter word DARTER
I've seen this crossword clue in The Daily Telegraph
D
A
R
T
E
R
More clues leading to the result DARTER
Buddy explains!
The straight part
We know that "diving bird" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case diving bird = darter.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 DARTER mean?
DARTER noun- One who darts, or who throws darts; that which darts.
- Any member of the family Anhingidae, waterbirds with long necks.
- Any of various darting freshwater fish of the family Percidae, that are usually small and brightly coloured and are native to North America.
- Any of the similar South American (and Panamanian) freshwater fish in the characin family Crenuchidae
- Any of the dragonflies in the genus Sympetrum
- Any of various hesperiid butterflies of the genera Arrhenes and Telicota.