The most common and most likely answer to this crossword clue is the 7 letter word LEANDER
I've seen this crossword clue in The Daily Telegraph
L
E
A
N
D
E
R
More clues leading to the result LEANDER
- Youth from Abydos with piercing look
- Hero's mate is slim, revolutionary communist
- Heroic lover with piercing sidelong look
- Hero’s lover’s name revealed in editorial
- Hero’s boyfriend died in comparatively poor surroundings
- Tragic Greek figure, skinny, next to the German
- Ill-fated lover‘s articles, two of them foreign
- Cross channel swimmer not covered in fat rejected warning to stop
- Hero’s beloved captain conceals name
- Boss around new L
Buddy explains!
The straight part
We know that "an enthusiastic swimmer (" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case an enthusiastic swimmer ( = leander.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.