Austere party for the pure at heart (4)
I believe the answer is:
'austere' is the definition.
'party for the pure at heart' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'for' means one lot of letters go next to another.
'at heart' indicates the central letters.
The central letters of 'pure' are 'ur'.
(Other definitions for dour that I've seen before include "Stern and severe looking" , "Gloomy-looking and forbidding" , "Severe and gloomy looking" , "Constituent parts" , "Sullen - unyielding" .)