Postscript about Greek letter used wrong way round in dialect (6)
I believe the answer is:
patois
'dialect' is the definition.
(I know that patois is a type of dialect)
'postscript about greek letter used wrong way round' is the wordplay.
'postscript' becomes 'ps' (abbreviation).
'about' is an insertion indicator.
'greek letter' becomes 'iota' (letter from the Greek alphabet).
'used wrong way round' says the letters should be written in reverse.
'iota' back-to-front is 'atoi'.
'ps' enclosing 'atoi' is 'PATOIS'.
'in' is the link.
(Other definitions for patois that I've seen before include "A regional dialect of a language" , "Informal local dialect" , "Local dialect heard in patios" , "'Argot, vernacular (6)'" , "speech" .)