Starts counting only dead fish (3)
I believe the answer is:
cod
'fish' is the definition.
(cod is a kind of fish)
'starts counting only dead' is the wordplay.
'starts' says to take the initial letters.
The first letters of 'counting only dead' is 'COD'.
(Other definitions for cod that I've seen before include "Cash on delivery (abbr.)" , "A foolish sort of a fish" , "North Atlantic fish" , "Not a real" , "Edible fish" .)