Fish drift off, mostly upside down! (4)
I believe the answer is:
eels
'fish' is the definition.
(eel is a type of fish)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for eels that I've seen before include "Water snakes twisting in lees" , "Smooth fishes" , "Else (anag.)" , "Slippery creatures" , "They have long bodies and very small fins" .)