Young fish constantly circling lake (5)
I believe the answer is:
elver
'young fish' is the definition.
(I've seen this before)
'constantly circling lake' is the wordplay.
'constantly' becomes 'ever' (I've seen this in another clue).
'circling' indicates putting letters inside (circling can mean going around or containing).
'lake' becomes 'L' (geographical abbreviation).
'ever' placed around 'l' is 'ELVER'.
(Other definitions for elver that I've seen before include "Immature fish" , "One may see one upriver" , "Young swimmer" , "Slippery youngster" , "Young slippery fish" .)