The most common and most likely answer to this crossword clue is the 11 letter word SHEARWATERS
I've seen this crossword clue in The Daily Telegraph
S
H
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A
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S
Buddy explains!

The straight part
We know that "Seabirds" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case seabirds = shearwaters.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 SHEARWATERS mean?
SHEARWATERS noun- Any of the long-winged pelagic seabirds of the family Procellariidae in genera Puffinus, Ardenna, and Calonectris, that breed on islands and coastal cliffs.
- (by extension) Any of the entire family Procellariidae, including certain of the petrels.