Swimmer using river always stopped by lake (5)
I believe the answer is:
elver
'swimmer' is the definition.
(elver is a kind of fish)
'always stopped by lake' is the wordplay.
'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' enclosing 'l' is 'ELVER'.
'using river' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for elver that I've seen before include "Young, migrating eel" , "Young slippery fish" , "Little fish" , "Young migratory fish" , "Young swimmer" .)