Register for a fish (4)
I believe the answer is:
bass
This clue is a double definition.
'register' is the first definition.
(vocal register)
'fish' is the second definition.
(bass is a kind of fish)
'for a' acts as a link.
(Other definitions for bass that I've seen before include "Fish; male singing voice" , "Deep-voiced male singer" , "Variety of sea and freshwater perch" , "Fish (which sings?)" , "Lowest male singer" .)