Of the countryside (6)
(Other definitions for rustic that I've seen before include "Characteristic of rural life" , "Like country life" , "Countryfied" , "Characteristic of country life" , "Of the countryside and country folk" .)
'of the countryside' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for rural that I've seen before include "Bucolic; pastoral" , "Sort of dean" , "Relating to country life" , "Connected with farming" , "Of the country - pastoral" .)