Fish round lake doing nothing (4)
I believe the answer is:
idle
'doing nothing' is the definition.
(I've seen this before)
'fish round lake' is the wordplay.
'fish' becomes 'ide' (type of fish).
'round' means one lot of letters goes inside another.
'lake' becomes 'L' (geographical abbreviation).
'ide' placed around 'l' is 'IDLE'.
(Other definitions for idle that I've seen before include "Turn over" , "Good-for-nothing" , "Lazy - unemployed" , "without purpose" , "Not active or at work" .)