Fat bit of sole for Eliza's fish (3)
I believe the answer is:
eel
'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 eel that I've seen before include "Moray or electric, for instance" , "Elongated fish" , "Snake-like sea creature" , "Slippery figure" , "Apode" .)