Mack maybe comes back for fish (3)
I believe the answer is:
eel
'fish' is the definition.
(eel is a type of fish)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for eel that I've seen before include "Slippery type" , "Snakelike fish" , "Evasive type" , "Electric swimmer, perhaps?" , "Apode" .)