Halibut ...
The name of a large flat fish beginning with "P" is a "Plaice."
Turbot
sprat
if this is to solve a clue from the guardian crossword the first letter is E and the fith is B
The fish you are referring to is likely a "sole." Soles are large, flat fish with a distinctive body shape and are commonly found in both freshwater and saltwater environments.
Skate. It's like a ray.
sole
halibut
Rays
ecas
a fish that lives in a flat
it could be either a mantaray or a stingray.