Young fish always crossing lake (5)
I believe the answer is:
elver
'young fish' is the definition.
(I've seen this before)
'always crossing lake' is the wordplay.
'always' becomes 'ever' (synonyms).
'crossing' indicates putting letters inside.
'lake' becomes 'l'.
'ever' going around 'l' is 'ELVER'.
(Other definitions for elver that I've seen before include "Immature fish" , "Immature eel" , "One may see one upriver" , "Young migratory fish" , "A baby eel" .)