The most common and most likely answer to this crossword clue is the 7 plus 5 letter word BASKING SHARK
I've seen this crossword clue in The Daily Telegraph
B
A
S
K
I
N
G
S
H
A
R
K
More clues leading to the result BASKING SHARK
- Leader in boat requesting quiet with ship following marine creature
- Big fish — try to catch majestic lot by old boat
- Swimmer requesting silence when there’s clamour around
- Graduates at college hear school member
- Swimmer hit, intercepted by men on board boat
- Great swimmer succeeded leader in party on ship
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 = basking shark.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.