Small fish always around lake (5)
I believe the answer is:
elver
'small fish' is the definition.
(term for a young eel)
'always around lake' is the wordplay.
'always' becomes 'ever' ('ever' can be a synonym of 'always').
'around' 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 "Young swimmer" , "slippery little thing" , "New slipper" , "Young slippery fish" , "Young, migrating eel" .)