Eel slipping off line -- this could be good for kippers (8,4)
I believe the answer is:
sleeping pill
'this could be good for kippers' is the definition.
(kip can mean to sleep)
'eel slipping off line' is the wordplay.
'off' indicates anagramming the letters.
'line' becomes 'l' (used when specifying particular lines from a poem).
'eel'+'slipping'='eelslipping'
'eelslipping' anagrammed gives 'sleepingpil'.
'sleepingpil'+'l'='SLEEPING-PILL'
(Other definitions for sleeping pill that I've seen before include "Medication for dropping off" , "Insomnia cure" , "Night-time tablet" , "Sleeplessness cure" , "Strong tranquilliser" .)