Fail to enunciate (4)
I believe the answer is:
slur
'fail to enunciate' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for slur that I've seen before include "Pronounce indistinctly - discredit" , "Pronounce indistinctly or speak ill of" , "Slight sound" , "Mumble words - it's an affront" , "It's not clear, so to speak" .)