Letter written in patois sent westwards (4)
I believe the answer is:
iota
'letter' is the definition.
(Greek letter)
'written in patois sent westwards' is the wordplay.
'written in' indicates the answer is hidden within the clue.
'westwards' says the letters should be written backwards (letters go right to left or from 'east' to 'west').
'atoi' can be found hidden inside 'patois sent'.
'atoi' written backwards gives 'IOTA'.
(Other definitions for iota that I've seen before include "Grain" , "Tiny bit" , "little part" , "Jot, particle" , "letter from abroad" .)