Crustacean, a sort confused with fish (8)
I believe the answer is:
ostracod
'crustacean' is the definition.
(ostracod is a kind of crustacean)
'a sort confused with fish' is the wordplay.
'confused' indicates an anagram.
'with' means one lot of letters go next to another.
'fish' becomes 'cod' (type of fish).
'a'+'sort'='asort'
'asort' with letters rearranged gives 'ostra'.
'ostra'+'cod'='OSTRACOD'
(Other definitions for ostracod that I've seen before include "Shellfish" , "Tiny crustacean" .)