Bit of jargon used on the third of October (5)
I believe the answer is:
argot
'bit of jargon' is the definition.
'argot' can be an answer for 'jargon' (argot can mean jargon or slang). I'm unsure of the 'bit of' bit.
'on the third of october' is the wordplay.
'on' says to put letters next to each other.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'third of october' becomes 'argo' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
't' after 'argo' is 'ARGOT'.
'used' is the link.
Can you help me to learn more?
(Other definitions for argot that I've seen before include "Special language of a groat?" , "secret language" , "Jargon - patois" , "Jargon or slang (of a group)" , "Patois, vernacular" .)