Language used when trouble started on Jason's ship (5)
I believe the answer is:
argot
'language' is the definition.
(argot can mean jargon or slang)
'trouble started on jason's ship' is the wordplay.
'started' says to take the initial letters.
'on' is a charade indicator (letters next to each other).
'jason's ship' becomes 'argo' (ship in Greek myth).
The first letter of 'trouble' is 't'.
't' after 'argo' is 'ARGOT'.
'used when' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for argot that I've seen before include "Jargon or slang (of a group)" , "secret language" , "Slang, patois" , "Special jargon of a group" , "Specialised language" .)