Fish slumber mostly upside down! (4)
I believe the answer is:
eels
'fish' is the definition.
(I've seen this before)
'slumber mostly upside down' is the wordplay.
'slumber' becomes 'sleep' ('sleep' can be a synonym of 'slumber').
'mostly' means to remove the last letter.
'upside down' says the letters should be written in reverse.
'sleep' with its final letter taken away is 'slee'.
'slee' reversed gives 'EELS'.
(Other definitions for eels that I've seen before include "Slithery fish" , "Sea creatures found in lees" , "They have long bodies and very small fins" , "Conger --; jellied --" , "Food for Cockney" .)