Oily fish (8)
I believe the answer is:
mackerel
'oily fish' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for mackerel that I've seen before include "that's fishy" , "Marine food-fish" , "Food fish with a blue-green back" , "Fish caught by spinner (or sprat?)" , "Oily food fish" .)