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 fish)
'underwear abandoning the lake' is the subsidiary indication.
'underwear' becomes 'lingerie' (lingerie is a kind of underwear).
'abandoning' indicates named letters should be taken away.
'the lake' becomes 'erie' (Lake Erie is one of the Canadian/US 'Great Lakes').
'lingerie' with 'erie' taken away is 'ling'.
'in' is the link.
(Other definitions for ling that I've seen before include "Heather - fish", "Common heather", "Cod-like fish", "Fish; heather", "Slender food fish", "Burbot", "A fish and a heather" and "Fish of the cod family".)