Regular portions of greasy fish (3)
I believe the answer is:
ray
'fish' is the definition.
(ray is a kind of fish)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for ray that I've seen before include "Sunbeam or flat fish" , "bit of sunshine" , "Second note for the flat fish" , "may be electric" , "Column of light on the fish?" .)