Register degree on ship (4)
I believe the answer is:
bass
'register' is the definition.
(vocal register)
'degree on ship' is the wordplay.
'degree' becomes 'ba' (Bachelor of Arts).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'ba'+'ss'='BASS'
(Other definitions for bass that I've seen before include "Variety of sea and freshwater perch" , "Large percussion instrument" , "Scottish rock" , "Performer" , "A fish, or the lowest pitch in music" .)