Fishy fellow, but he shines (3)
I believe the answer is:
ray
'fishy' is the definition.
(type of fish)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for ray that I've seen before include "Beam or flatfish" , "Narrow beam of light" , "Flat fish or sun beam" , "bit of sunshine" , "Sunbeam or flat fish" .)