Swimmer always found across lake (5)
I believe the answer is:
elver
'swimmer' is the definition.
(elver is a kind of fish)
'always found across lake' is the wordplay.
'always' becomes 'ever' ('ever' can be a synonym of 'always').
'found across' means one lot of letters goes inside another.
'lake' becomes 'l'.
'ever' placed around 'l' is 'ELVER'.
(Other definitions for elver that I've seen before include "A young 11" , "One may see one upriver" , "Young swimmer" , "New slipper" , "Slippery youngster" .)