The most common and most likely answer to this crossword clue is the 5 letter word GREBE
I've seen this crossword clue in The Times
G
R
E
B
E
More clues leading to the result GREBE
- Swimmer seen among icebergs heading west
- Bird black in colour? Not completely!
- Curt welcome baffles British diver
- Bird in 24, bees every second buzzing off
- Bird nesting in icebergs going north
- Go mostly around capital of Belgium
- Bird, when nesting on Channel Islands, turns into shipping hazard!
- Go without duck, mostly one revolting bird
- Aquatic bird I see leaving frozen mass, going north
- Understanding finally about live bird
Buddy explains!

The straight part
We know that "Swimmer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case swimmer = grebe.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 GREBE mean?
GREBE noun- Any of several waterbirds in the cosmopolitan family Podicipedidae. They have strong, sharp bills, and lobate toes.