Slight legato effect (4)
I believe the answer is:
slur
This clue is a double definition.
'slight' is the first definition.
(I've seen this before)
'legato effect' is the second definition.
(I've seen this in another clue)
(Other definitions for slur that I've seen before include "Stigma" , "Slight, insult" , "Contentious allegation" , "It's not clear, so to speak" , "making indistinct sound" .)