Young swimmer always crossing lake (5)
I believe the answer is:
elver
'young swimmer' is the definition.
(I've seen this before)
'always crossing lake' is the wordplay.
'always' becomes 'ever' (synonyms).
'crossing' means one lot of letters goes inside another (to cross can mean to go over or around).
'lake' becomes 'L' (geographical abbreviation).
'ever' placed around 'l' is 'ELVER'.
(Other definitions for elver that I've seen before include "Young migratory fish" , "A baby eel" , "Slippery youngster" , "A young 11" , "One may see one upriver" .)