Slow left on fantastic ship (5)
I believe the answer is:
largo
'slow' is the definition.
('largo' can be similar in meaning to 'slow')
'left on fantastic ship' is the wordplay.
'left' becomes 'l' (common abbreviation).
'on' says to put letters next to each other.
'fantastic ship' becomes 'argo' (**).
'l'+'argo'='LARGO'
(Other definitions for largo that I've seen before include "Handel's piece?" , "Musically in slow tempo and dignified style" , "Slow and dignified tempo in music" , "Mus. slow movement" , "In a stately manner, musically" .)