Austere party for the pure at heart (4)
I believe the answer is:
dour
'austere' is the definition.
(thesaurus)
'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'.
'do'+'ur'='DOUR'
(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" .)