Swimmer in underwear abandoning the lake (4)
I believe the answer is:
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).
(Other definitions for ling that I've seen before include "Fish; heather", "Common heather", "Heather - fish", "A fish and a heather", "Burbot", "Slender food fish", "Swimmer", "Cod-like fish" and "Fish of the cod family".)