Fish, dead one (4)
I believe the answer is:
dace
'fish' is the definition.
(dace is a kind of fish)
'dead one' is the wordplay.
'dead' becomes 'd'.
'one' becomes 'ace' ('ace' can be a synonym of 'one'**).
'd'+'ace'='DACE'
(Other definitions for dace that I've seen before include "Fish in the carp family" , "Freshwater fish" , "Chubby swimmer" , "Small river fish" .)