Jargon is about messages (7)
I believe the answer is:
signals
'messages' is the definition.
(I've seen this before)
'jargon is about' is the wordplay.
'jargon' becomes 'slang' (synonyms).
'about' says the letters should be written backwards.
'slang'+'is'='slangis'
'slangis' in reverse letter order is 'SIGNALS'.
(Other definitions for signals that I've seen before include "Coded communications" , "Actions that encode messages" , "Flags, gestures" , "(Traffic) lights" , "Indications or gestures as information or warning" .)