Initially pretty averse to one’s local dialect (6)
I believe the answer is:
patois
'local dialect' is the definition.
(I've seen this before)
'initially pretty averse to one's' is the wordplay.
'initially' says to take the initial letters.
'one's' becomes 'is' (roman numeral).
The first letters of 'pretty averse' is 'pa'.
'pa'+'to'+'is'='PATOIS'
(Other definitions for patois that I've seen before include "speech" , "manner of speaking" , "'Argot, vernacular (6)'" , "Language of group" , "Spoken regional dialect" .)