Young swimmer always seen around lake (5)
I believe the answer is:
elver
'young swimmer' is the definition.
(I've seen this before)
'always seen around lake' is the wordplay.
'always' becomes 'ever' (synonyms).
'seen around' indicates putting letters inside.
'lake' becomes 'L' (geographical abbreviation).
'ever' placed around 'l' is 'ELVER'.
(Other definitions for elver that I've seen before include "A baby eel" , "Young migratory fish" , "Immature fish" , "Young, migrating eel" , "Immature eel" .)