Register a fish (4)
I believe the answer is:
bass
This clue is a double definition.
'register' is the first definition.
(vocal register)
'a fish' is the second definition.
(bass is a kind of fish)
(Other definitions for bass that I've seen before include "Variety of sea and freshwater perch" , "Freshwater fish and lowest voice" , "Sea perch - male voice" , "Fish - low voice" , "Fish; male singing voice" .)