Large edible flatfish (7)
I believe the answer is:
halibut
'large edible flatfish' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for halibut that I've seen before include "Marine food fish" , "One's scaled" , "Large flatfish, tasty to eat" , "Utah lib (anag)" , "Something fishy here" .)