Sluggish fish circling lake (4)
I believe the answer is:
idle
'sluggish' is the definition.
(thesaurus)
'fish circling lake' is the wordplay.
'fish' becomes 'ide' (type of fish).
'circling' means one lot of letters goes inside another (circling can mean going around or containing).
'lake' becomes 'L' (geographical abbreviation).
'ide' going around 'l' is 'IDLE'.
(Other definitions for idle that I've seen before include "not working in > lounge" , "Inactive - lazy" , "Lazy - vain" , "At leisure" , "Member of comedy team" .)