Slow learner on Jason's ship (5)
I believe the answer is:
largo
'slow' is the definition.
(musical term)
'learner on jason's ship' is the wordplay.
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'jason's ship' becomes 'argo' (ship in Greek myth).
'l'+'argo'='LARGO'
(Other definitions for largo that I've seen before include "To be played slowly and broadly" , "Musical piece" , "major key" , "Slow and dignified tempo in music" , "Slow and dignified, like Handel's composition" .)