Fellow leaves first of two shellfish (5)
I believe the answer is:
ormer
'shellfish' is the definition.
(ormer is a kind of shellfish)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for ormer that I've seen before include "Edible abalone; rhymes with projecting window" , "Sea-ear, an abalone found near the Channel Is" , "Ear-shaped shell" , "Seafood" , "Delicacy from Channel Islands" .)