Rinsed a thrashing herring (7)
I believe the answer is:
sardine
'herring' is the definition.
(sardine is a kind of herring)
'rinsed a thrashing' is the wordplay.
'thrashing' indicates an anagram (I've seen this in other clues).
'rinsed'+'a'='rinseda'
'rinseda' anagrammed gives 'SARDINE'.
(Other definitions for sardine that I've seen before include "Fish in tin?" , "Young pilchard" , "Swimmer" , "(Cramped?) fish" , "Fish (often tinned)" .)