Register fish (4)
I believe the answer is:
bass
I believe this clue is a double definition.
'register' is the first definition.
(vocal register)
'fish' is the second definition.
(bass is a kind of fish)
(Other definitions for bass that I've seen before include "Freshwater fish and lowest voice" , "A fish, or the lowest pitch in music" , "Lowest register of the male voice" , "Scottish rock" , "Male singer and saltwater fish" .)