Fish that nearly all upset sleep (4)
I believe the answer is:
eels
'fish that nearly all upset sleep' is the definition.
'eels' can be an answer for 'fish' (eel is a type of fish). I'm not certain of the remainder of the definition.
'fish that nearly all upset sleep' is the wordplay.
'fish' becomes 'eel' (eel is a type of fish).
'that nearly all upset sleep' becomes 's' (I can't explain this - if you can you should give a lot more credence to this answer).
'eel'+'s'='EELS'
Can you help me to learn more?
(Other definitions for eels that I've seen before include "Long, fleshy fish" , "Long fishes" , "Long thin fish" , "Jellied --, traditional dish" , "Food for Cockney" .)