Comparatively inactive fish seen round lake and river (5)
I believe the answer is:
idler
'comparatively inactive' is the definition.
('idle' can be similar in meaning to 'inactive')
'fish seen round lake and river' is the wordplay.
'fish' becomes 'ide' (type of fish).
'seen round' indicates putting letters inside.
'lake' becomes 'L' (geographical abbreviation).
'and' is a charade indicator (letters next to each other).
'river' becomes 'r' (abbreviation for river).
'ide' placed around 'l' is 'idle'.
'idle'+'r'='IDLER'
(Other definitions for idler that I've seen before include "One avoiding work" , "One who doesn't want to work" , "Wastrel; The ---, Dr Johnson essays" , "Lazybones; gear wheel" , "Johnson's work" .)