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