When a trout swims drunkenly around Beachy Head, it's definitely fishy! (6)
I believe the answer is:
turbot
I'm afraid I can't even guess how this answer relates to the clue.
Can you help me to learn more?
(Other definitions for turbot that I've seen before include "Species of flatfish" , "Species of fish" , "Edible flatfish" , "Large food fish" , "Large brownish European flatfish" .)