Jargon a king understood (5)
I believe the answer is:
argot
'jargon' is the definition.
(argot can mean jargon or slang)
'a king understood' is the wordplay.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'understood' becomes 'got' (getting is a kind of understanding).
'a'+'r'+'got'='ARGOT'
(Other definitions for argot that I've seen before include "Father or mother" , "Slang, patois" , "Patois for groat" , "Special language of a groat?" , "Slang of a particular group, originally thieves" .)