Little fish constantly circling lake (5)
I believe the answer is:
elver
'little fish' is the definition.
(young eel)
'constantly circling lake' is the wordplay.
'constantly' becomes 'ever' (I've seen this before).
'circling' is an insertion indicator (circling can mean going around or containing).
'lake' becomes 'L' (geographical abbreviation).
'ever' enclosing 'l' is 'ELVER'.
(Other definitions for elver that I've seen before include "New slipper" , "A baby eel" , "Young swimmer" , "Immature fish" , "Young migratory fish" .)