Crossword Genius

Swimmer in underwear abandoning the lake (4)


I believe the answer is:


I'm a little stuck... Click here to teach me about this clue!

Here is my best explanation:

'swimmer' is the definition.
(ling is a kind of aquatic vertebrate)

'in underwear abandoning the lake' is the wordplay.
I cannot quite see how this works, but
'in' is within the answer.
'lake' could be 'l' and 'l' is located in the answer.
A single letter 'g' remains which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).

Can you help me to learn more?