Young swimmer constantly circling lake (5)
I believe the answer is:
elver
'young swimmer' is the definition.
(I've seen this before)
'constantly circling lake' is the wordplay.
'constantly' becomes 'ever' (I've seen this in another clue).
'circling' means one lot of letters goes inside another (circling can mean going around or containing).
'lake' becomes 'L' (geographical abbreviation).
'ever' enclosing 'l' is 'ELVER'.
(Other definitions for elver that I've seen before include "A baby eel" , "A young 11" , "Slippery youngster" , "slippery little thing" , "Young slippery fish" .)