Swimmer using river always stopped by lake (5)
I believe the answer is:
Here is my best explanation:
'swimmer' is the definition.
(elver is a kind of fish)
'always stopped by lake' is the subsidiary indication.
'always' becomes 'ever' ('ever' can be a synonym of 'always').
'stopped by' is an insertion indicator (I've seen this in other clues).
'lake' becomes 'l'.
'ever' placed around 'l' is 'elver'.
'using river' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
(Other definitions for elver that I've seen before include "young swimmer", "Youngster in Anguilla", "Young migratory fish", "A young eel", "Immature eel", "slippery little thing", "Slippery youngster", "A baby eel", "Little fish" and "Young slippery fish".)