Jason's craft heads north in the atmosphere (5)
I believe the answer is:
argon
'in the atmosphere' is the definition.
(argon makes up part of the atmosphere)
'jason's craft heads north' is the wordplay.
'jason's craft' becomes 'argo' (I've seen this in another clue).
'heads' says to take the initial letters.
The initial letter of 'north' is 'n'.
'argo'+'n'='ARGON'
(Other definitions for argon that I've seen before include "Inert gas used in light bulbs" , "Inert gas, present in small amounts in the air" , "Noble gas, Ar" , "elementary" , "noble element" .)