Slanderous attacks over captain's fish (10)
I believe the answer is:
mudskipper
'fish' is the definition.
(mudskipper is a kind of fish)
'slanderous attacks over captain's' is the wordplay.
'slanderous attacks' becomes 'mud' (as in dragging someone's name through the mud).
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'captain' becomes 'skipper' (synonyms).
'mud'+'skipper'='MUDSKIPPER'
(Other definitions for mudskipper that I've seen before include "Swimmer" , "Tropical fish that can move over land - risked pump (anag)" .)