Jargon of stars incoherent ultimately (5)
I believe the answer is:
argot
'jargon' is the definition.
(argot can mean jargon or slang)
'stars incoherent ultimately' is the wordplay.
'stars' becomes 'argo' (I've seen this before).
'ultimately' indicates one should take the final letters.
The final letter of 'incoherent' is 't'.
'argo'+'t'='ARGOT'
'of' acts as a link.
(Other definitions for argot that I've seen before include "Cant" , "Specialised language" , "Father or mother" , "Ancient Greek coin" , "Special language of a groat?" .)