Lazy leaving line in fish (4)
I believe the answer is:
idle
'lazy' is the definition.
('idle' can be similar in meaning to 'lazy')
'line in fish' is the wordplay.
'line' becomes 'l'.
'in' is an insertion indicator.
'fish' becomes 'ide' (I've seen this before**).
'l' going inside 'ide' is 'IDLE'.
'leaving' is the link.
(Other definitions for idle that I've seen before include "Good-for-nothing" , "Unoccupied or not in use" , "Loaf" , "Not used to work" , "Workshy" .)