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' means one lot of letters go next to another.
'upset' indicates anagramming the letters.
'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" .)