Sleeper cured fish (6)
I believe the answer is:
kipper
This clue is a double definition.
'sleeper' is the first definition.
(kip can mean sleep)
'cured fish' is the second definition.
(I've seen this before)
(Other definitions for kipper that I've seen before include "Dried and smoked herring" , "One yet to wake" , "Cured fish (one having forty winks?)" , "part of breakfast?" , "Cured or smoked herring" .)