Interfere with day, getting purist upset (7)
I believe the answer is:
disrupt
'interfere with' is the definition.
(thesaurus)
'day getting purist upset' is the wordplay.
'day' becomes 'd' (abbreviation for day).
'getting' is a charade indicator (letters next to each other).
'upset' is an anagram indicator.
'purist' anagrammed gives 'isrupt'.
'd'+'isrupt'='DISRUPT'
(Other definitions for disrupt that I've seen before include "Interrupt and spoil" , "Disturb" , "Break up" , "Throw into disorder" .)